As a strategy, it seems to make sense: Secure the budding superstar before he becomes prohibitively expensive. It worked out great for Real Madrid with Vinícius Júnior (acquired for 45m at age 16) and Rodrygo (also 45m at age 17), less so with Reiner (30m at 18). And, of course, the jury is still out on Endrick, who completed his move to LaLiga this summer.

This past summer, we already saw Endrick, Leny Yoro, Archie Gray, Yankuba Minteh, Ernest Nuamah and Savinho move for 25m or more. What they have in common is that none of them have turned 21, and most of them have a very small body of work: a season, maybe two, as a starter in a top league.

The logic is simple: Get a player like Yoro while he's young and you'll get more years out of him, plus he'll cost you less than three or four years down the line when he's an established superstar. The counterargument, however, is just as familiar. When dealing with very young players, there are tons of unknowns, tons of things that can go wrong and a limited amount of data on which to base your assessment. It's a bit like the No. 1 pick in the NBA draft. Yes, you can land a generational talent, or you could end up with Anthony Bennett, Kwame Brown or Michael Olowokandi.

With this in mind, I decided to look at the 25 most expensive transfer of Under-21 players between 1999 and 2022, figuring it was too early to assess those who moved last summer. I adjusted for inflation using this handy tool, and we used figures from Transfermarkt for consistency. Note that these figures aren't "official" because fees generally aren't disclosed, but they're generally reliable estimates. Note too that in some cases -- like Vinícius and Rodrygo, who remained in Brazil after Real Madrid announced their arrivals, or Kylian Mbappé, who technically joined PSG initially on loan -- we went with the date they were signed rather than when they actually transferred.

Here's what the numbers say.

1. Kylian Mbappé

Position: Forward
Move: Monaco to Paris Saint-Germain (18 years old in 2017)
Fee: 180m (Adjusted for inflation: 210.6m)

The granddaddy of them all, PSG would've wanted to keep him rather than losing him as a free agent to Real Madrid, but they got as much as they could out of him when there.

Verdict: Met expectations

2. Ousmane Dembélé

Position: Forward
Move: Borussia Dortmund to Barcelona (20, 2017)
Fee: 145m (169.7m)

Barca obviously overpaid massively to sign him because they were sitting on the Neymar money. Injuries curtailed his contribution (85 LaLiga starts in six years) and they had to let him go for 50m to PSG in 2023.

Verdict: A bust, at least for the team that signed him

3. João Félix

Position: Forward
Move: Benfica to Atletico Madrid (19, 2019)
Fee: 127.2m (144m)

His supposedly unlimited upside explains the huge fee, but he never quite went to the next level and has bounced around on loan the past few years instead.

Verdict: A bust, at least for the team that signed him

4. Matthijs de Ligt

Position: Defender
Move: Ajax to Juventus (19, 2019)
Fee: 85.5m (96.8m)

The Dutch international faced a steep learning curve at Juve, but he did his part and his club recouped most of the money when they moved him on to Bayern Munich in 2022.

Verdict: Met expectations

5. Raheem Sterling

Position: Forward
Move: Liverpool to Manchester City (20, 2015)
Fee: 63.7m (76m)

A huge part of Pep Guardiola's early dominant City sides, the club got a big fee for him when he moved to Chelsea in 2022.

Verdict: Surpassed expectations

6. Christian Pulisic

Position: Forward
Move: Borussia Dortmund to Chelsea (20, 2019)
Fee: 64m (72.4m)

Injuries played a part in his struggles as he never established himself as a consistent starter at Stamford Bridge. Moved to Milan for a cut-price 22m and is doing much better now.

Verdict: A bust, at least for the team that signed him

7. Anthony Martial

Position: Forward
Move: AS Monaco to Manchester United (19, 2015)
Fee: 60m (71.6m)

Spent nine years at Old Trafford, starting more than half the club's league games in a season just twice, and leaving as a free agent. Showed glimpses of his quality, but no consistency.

Verdict: A bust, at least for the team that signed him

8. Leroy Sané

Position: Forward
Move: Schalke to Manchester City (20, 2016)
Fee: 52m (61.9m)

Three mostly successful years were followed by a serious injury and then, somehow, a move to Bayern that enabled City to recoup their investment. Still, he exited with two Premier League titles, an FA Cup and three League Cups.

Verdict: Met expectations

9. Nicolas Anelka

Position: Forward
Move: Arsenal to Real Madrid (20, 1999)
Fee: 35m (56.2m)

Scored just two league goals and had many arguments with the manager in his one season at the Bernabeu. Despite Champions League play and the fact Madrid recouped a big chunk of his fee, this is still a "miss."

Verdict: A bust, at least for the team that signed him

10. Rafael Leão

Position: Forward
Move: Lille to Milan (20, 2019)
Fee: 49.5m (56m)

Endured a slow start to his Milan career, but has grown steadily since. He's still waiting to take that leap to the top tier, but so far, so good.

Verdict: Met expectations

11. Javier Saviola

Position: Forward
Move: River Plate to Barcelona (19, 2001)
Fee: 35.9m (55.1m)

His Camp Nou career started strong, but petered out in Year 3, followed by loan spells and free agency.

Verdict: A bust, at least for the team that signed him

12. Wayne Rooney

Position: Forward
Move: Everton to Manchester United (18, 2004)
Fee: 37m (53.2m)

Played 13 seasons at Old Trafford, won plenty of trophies and retired as the club's all-time goalscorer. Enough said.

Verdict: Surpassed expectations

13. Vinicius Jr.

Position: Forward
Move: Flamengo to Real Madrid (16, 2017)
Fee: 45m (52.6m)

Took a while to get going, but the Brazil star has gone from strength to strength over the past three seasons and is very much a leader at the Bernabeu.

Verdict: Surpassed expectations

14. Rodrygo

Position: Forward
Move: Santos to Real Madrid (17, 2018)
Fee: 45m (51.7m)

Established himself as a starter at 21 and has only gotten better since.

Verdict: Met expectations

15. Lucas Moura

Position: Forward
Move: Sao Paulo to Paris Saint-Germain (20, 2012)
Fee: 40m (48.6m)

Never quite achieved superstardom, but he was a big part of PSG's early success under Qatari ownership and they got back most of what they paid for him when he moved on to Tottenham after five years.

Verdict: Met expectations

16. Fábio Silva

Position: Forward
Move: Porto to Wolverhampton Wanderers (18, 2020)
Fee: 40m (44.9m)

Had started one league game for Porto when Wolves picked him up in 2020. Since then, he's had three loan spells away from the club and caused plenty of head-scratching.

Verdict: A bust, at least for the team that signed him

17. Luke Shaw

Position: Defender
Move: Southampton to Manchester United (18, 2014)
Fee: 37.5m (44.8m)

On the one hand, he has had 10 years at United. On the other, it has been one of the worst decades in the club's history and injuries have limited him to starting less than half their league games since he arrived.

Verdict: A bust, at least for the team that signed him

18. Dejan Kulusevski

Position: Forward
Move: Atalanta to Juventus (19, 2019)
Fee: 39m (44.1m)

Very inconsistent at Juventus, the mitigating factor here is that they eventually got most of their fee back when they shifted him to Tottenham.

Verdict: Met expectations

19. Renato Sanches

Position: Midfielder
Move: Benfica to Bayern Munich (18, 2016)
Fee: 35m (41.7m)

A move that was too much, too soon for a player who never made the grade, whether at Bayern or elsewhere (Swansea City on loan, Lille, PSG, Roma or Benfica).

Verdict: A bust, at least for the team that signed him

20. Nuno Mendes

Position: Defender
Move: Sporting Club to Paris Saint-Germain (19, 2021)
Fee: 38m (41.5m)

Was developing nicely until injury cost him most of last season. Still 22, he's now on a trajectory to justify his fee.

Verdict: Met expectations

21. Josko Gvardiol

Position: Defender
Move: Dinamo Zagreb to RB Leipzig (18, 2020)
Fee: 36.8m (40.2m)

Has already established as one of the best defenders in the world, and Leipzig made nearly tripled their money in transferring him to Manchester City.

Verdict: Surpassed expectations

22. Jules Koundé

Position: Defender
Move: Bordeaux to Sevilla (19, 2019)
Fee: 35m (39.6m)

Enjoyed three solid seasons at Sevilla and then made a move to Barcelona that enabled his first LaLiga club to make a tidy profit.

Verdict: Surpassed expectations

23. Dominik Szoboszlai

Position: Midfielder
Move: FC Salzburg to RB Leipzig (20, 2021)
Fee: 36m (39.3m)

One of those "in-house" moves between Red Bull clubs, so it needs to be taken with a pinch of salt. That said, he made the grade at Leipzig and moved to Liverpool for nearly twice what it cost to get him to the German Bundesliga initially.

Verdict: Surpassed expectations

24. Wesley Fofana

Position: Defender
Move: Saint-Etienne to Leicester City (19, 2020)
Fee: 35m (39.3m)

Enjoyed an excellent debut season at Leicester, followed by a serious injury (which wouldn't be his last). Transferred to Chelsea for twice what they paid for him, making him a success for Leicester, though his long-term status remains unclear.

Verdict: Surpassed expectations

25. Amadou Onana

Position: Midfielder
Move: Lille to Everton (20, 2022)
Fee: 35m (35m)

Had two very good seasons for a struggling club followed by, this summer, a 50m transfer to Aston Villa.

Verdict: Met expectations

You might or might not agree with the ratings, but the thing to remember here is whether the deal worked out for the original club, not whether the player went on to excel elsewhere. In some cases, even after subpar performances, fetching a good transfer fee for the player was enough to salvage the original deal.

What conclusions can we draw? Well, you can leave Mbappe to one side: he's an outlier. From a pure bean-counting perspective, given how much he cost and the fact he left for nothing in midcareer, you can call it a miss. But given PSG's financial situation -- in other words, the fact they could seemingly spend freely -- and the need to get on the map, you can possibly justify it.

Beyond that, it's telling how many more misses than hits there are in the top 10. The "Dembele folly" -- with Barca panicking after Neymar's departure -- is well-chronicled. Joao Felix was a moon shot: he boasted a unique set of skills and probably a higher ceiling than anyone on this list (other than Mbappe), but for that deal to work out, you would have needed him to ascend to GOAT candidate or thereabouts. Meanwhile De Ligt is the only defender in the top 16, which suggests it's pretty evident clubs feel more confident splashing cash for young attacking prospects.

Once you get under a certain fee -- say, 55m -- there are more "hits." Though it's telling that several (Onana, Fofana, Szoboszlai, Kounde, Gvardiol) resulted in midsized clubs rolling the dice and quickly transferring the player to a bigger club within a couple seasons. That seems to be the sweet spot for that sort of deal.

Another point that jumps out is longevity and how that affects perception. Here, it's all about Manchester United with Rooney, Shaw and Martial staying for 13, 10 (and counting) and 9 years, respectively.

Rooney was an unqualified success obviously, but to what degree is our perception of the other two negatively affected by longevity? Hindsight is 20/20. If we'd compiled this list in 2020, following a year in which Martial was fit and scored 23 goals, we might rate him higher and suggest he finally turned the corner. In Shaw's case, we could say that his fee -- spread out over 10 years of service -- represented value for money. (Though, on the other hand, his injury-record means United had to pay other guys to fill the left-back role during his long injury layoffs too.)

The longevity argument works in reverse too, by the way. Had Leicester hung on to Fofana -- given how unlucky he's been with injuries -- you would probably call his move a failure. The same could be said of De Ligt and Kulusevski at Juventus.

The bottom line? You're rolling the dice when you spend big on young players, simply because they're more difficult to assess. If you're a bigger club, it helps to have the patience to bring them along slowly, like Real Madrid did with Vini and Rodrygo or Milan with Leao, but there's no real formula here. Rarely does it make sense to go really, really big because if you do that, the ceiling needs to be really, really high. Most of the time, it simply isn't.