We looked previously at players who move for large sums before their 21st birthday, but what happens when the cohort is even younger? What happens when the players have yet to turn 19 and, in most cases, there is even less of a body of work to go on? A season, maybe two, in the pros -- if you're lucky -- though often far less?

So here are the 25 most expensive transfers of Under-19 players between 1999 and 2022 -- again, we left out those who moved in the last two seasons, figuring they'd take more time to assess. We also adjusted for inflation using 2022 as a benchmark with this handy tool, and we used figures from Transfermarkt for consistency. Note that these figures aren't "official" because fees generally aren't disclosed, though they can be considered reliable estimates.

Note also that some players -- Pedri and Tino Livramento are two examples -- didn't make the list because while they ended up costing their acquiring club plenty, (25m-plus, in both cases) this only happened after they fulfilled various appearances and transfer bonuses, which in a sense meant the downside was limited.

The first nine guys also feature on the other list, and you're reminded of two things: clubs will break the bank if they think they have a potential generational talent and yeah, the Kylian Mbappé deal was totally out-of-kilter with market rates, especially juxtaposed against his peers. (His deal was worth more in inflation adjusted Euros than the next four guys on the list combined.)

1. Kylian Mbappé

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

The grand-daddy 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. 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

3. Vinícius Júnior

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

4. 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 then.

Verdict: Met expectations

5. Antonio Cassano

Position: Forward
Move: Bari to Roma (18, 2001)
Fee: 31m (47.6m)

A tricky one to rate. He had some excellent seasons, but fell out badly with club and ended up leaving for Real Madrid for just 5.5m six months before free agency. That knocks him down.

Verdict: Bust

6. 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 been on three different loan spells and has caused plenty of head-scratching among Wolves fans.

Verdict: Bust

7. Luke Shaw

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

On the one hand, he's had 10 years at United. On the other, it's 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: Bust

8. 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: Bust

9. 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

10. Alessandro Bastoni

Position: Defender
Move: Atalanta to Inter (18, 2017)
Fee: 31.6m (37m)

Despite having just three Serie A appearances under his belt when Inter signed him, the gamble proved worthwhile as he's now one of the top centre backs in Europe.

Verdict: Met expectations

11. William Saliba

Position: Defender
Move: Saint Etienne to Arsenal (18, 2019)
Fee: 30m (34m)

A bit like Bastoni, he was sent on loan to develop for several seasons before breaking into the first team. Today he's a key starter for the Gunners.

Verdict: Met expectations

12. Eduardo Camavinga

Position: Midfielder
Move: Rennes to Real Madrid (18, 2021)
Fee: 31m (33.9m)

A Ligue 1 starter at 16, Camavinga had a significant body of work before he was signed by Madrid. Quickly established himself as part of the midfield rotation.

Verdict: Met expectations

13. Reiner

Position: Forward
Move: Flamengo to Real Madrid (18, 2020)
Fee: 30m (33.7m)

He's endured three loan stints, at ever-smaller clubs, with a total of just 20 starts in four years. Reinier is the cautionary yang to the yin of Vini and Rodrygo.

Verdict: Bust

14. Alexandre Pato

Position: Forward
Move: Internacional to Milan (17, 2007)
Fee: 24m (32.3m)

An absolute sensation in his first four seasons, a series of bad injuries curtailed his progress after that. Nevertheless, Milan recouped more than half their fee when he left for Corinthians.

Verdict: Met expectations

15. Sergio Aguero

Position: Forward
Move: Independiente to Atletico Madrid (17, 2006)
Fee: 23m (31.7m)

A precocious talent in Argentina for Independente at 16, he quickly established himself as a devastating scorer in LaLiga before moving to Manchester City for a sizable fee.

Verdict: Surpassed expectations

16. Jude Bellingham

Position: Midfielder
Move: Birmingham City to Borussia Dortmund (17, 2020)
Fee: 27.7m (31.1m)

The England star took the transition from Championship to Bundesliga in stride, establishing himself as a regular and then fetching a whopping 100m fee when he moved to Madrid.

Verdict: Surpassed expectations

17. Jérémy Doku

Position: Forward
Move: Anderlecht to Rennes (18, 2020)
Fee: 26m (29.2m)

Had his fair share of ups and downs at Rennes in terms of performance, but his skills were evident and the club were able to move him for more than twice what they paid for him less than three years later.

Verdict: Met expectations

18. Stephan El Shaarawy

Position: Forward
Move: Genoa to Milan (18, 2011)
Fee: 20.3m (25.3m)

Had his moments in Milan, including one memorable campaign, but injuries and instability at the club meant he never really made his mark.

Verdict: Bust

19. Pietro Pellegri

Position: Forward
Move: Genoa to AS Monaco (16, 2018)
Fee: 20.9m (24m)

Serie A's youngest-ever debutant at the time made a huge move to Monaco, but injuries hampered his development and he failed to make a single league start in three-and-a-half seasons.

Verdict: Bust

20. Amad

Position: Forward
Move: Atalanta to Manchester United (18, 2020)
Fee: 21.3m (23.9m)

He may yet come good, but his development at United was slowed by injury and inconsistency and he's had to go on loan twice. Worth noting too that he'd played just 63 minutes of first-team football when he was signed.

Verdict: Bust

21. Willem Geubbels

Position: Forward
Move: Lyon to AS Monaco (16, 2018)
Fee: 20m (23m)

Made two first team appearances for Lyon before his big move to Monaco, which was marked by a nightmarish series of injuries. Left the club on a free transfer in January 2023.

Verdict: Bust

22. Cristiano Ronaldo

Position: Forward
Move: Sporting to Manchester United (18, 2003)
Fee: 15m (22m)

You probably don't need me to tell you this one worked out pretty well. He scored tons of goals, won tons of trophies and fetched tons of money when he moved to Real Madrid.

Verdict: Surpassed expectations

23. Paulinho

Position: Forward
Move: Vasco da Gama to Bayer Leverkusen (17, 2018)
Fee: 18.5m (21.2m)

Struggled to adapt to the Bundesliga and then suffered a cruciate injury that cost him an entire season. Unable to make much of an impact before leaving on a free transfer for Atletico Mineiro.

Verdict: Bust

24. Mathys Tel

Position: Forward
Move: Rennes to Bayern Munich (17, 2022)
Fee: 20m (20m)

It's still early, but he's on the right trajectory, having made more than 50 first-team appearances before his 19th birthday.

Verdict: Met expectations

25. Valeri Bojinov

Position: Forward
Move: Lecce to Fiorentina (18, 2005)
Fee: 14m (19.7m)

Made his pro debut in Serie A at 15, reached double digit goals by January at 18 and the latter accomplishment got him his big move. Plagued by inconsistency and injuries, he would go on to move no fewer than 21 times in his career.

Verdict: Bust

There's a obviously a ton of variance here, as evidenced by the fact that Geubbels and Cristiano sit next to each other on the list. But I was somewhat surprised to see at least six outright hits (Rooney, Ronaldo, Gvardiol, Bellingham, Aguero, Vinícius) to go with the ones who at least met expectations (and could turn into elite players as their career progresses). When you consider that while you may be paying a high to sign an 18-year-old, odds are the initial salary will be far lower than for a guy in mid-career at the same transfer fee, you can understand clubs' willingness to roll the dice.

Of the guys who came up short, two things stand out. With younger players, you don't know who is going to be more or less injury prone and injuries at a younger age can be more debilitating. The other is that the extent of a player's body of work matters. It's probably not a coincidence that several of those rated as busts (Fabio Silva, Reiner, Diallo) simply had very few senior minutes on the pitch at the time of their move.

Then there's the obvious wild card: you just don't know how a player's personality is going to develop. Cassano was a teenage sensation: could anyone have predicted his future self-destructive behaviour? That's the million-dollar question.