But I don't think a 21/22 year old is at their peak either.
Bellingham is a player that moved to Dortmund from Birmingham at the age of 16 - I don't see him as something that regularly happens in the transfer window and is more of a one off.
100 million for an 18 year old is alot, but I don't nearly see 21 or 22 year old as that far away either.
Liverpool Players like Salah, Luis Diaz, Minamino, Thiago, Allison, Fabinho, Shaqiri, Van dijk, Widjnaldum. Were all 24/25>
Chelsea players like Kante, Lukaku, Niguez, Werner, Ziyech, Mendy, Thiago silva, Kovacic, Jorginho, Morata, Drinkwater, Rudiger, Giroud, Luiz Higuain. Were all 24/25>
Like I can see your point in targeting players in their prime or close to their prime like a season before -
But I personally don't see why going for a 18 year old is bad in your eyes but a 21/22 year old is a good transfer. They still have a lot of experience they have to gain to hit their prime which means the likelihood of them coming and controlling their performances to a top high standards at the age of 21 or 22 is low, especially with heavy transfer fees above their forehead.
I even look at Bruno fernandes who is arguably our best player, his was bought at his peak than at a younger age which shows was a time he did struggle. Salah and de Bryune both struggled at a younger age in the PL and arguably became the best players of the PL after returning in their prime.