Maybe he reached his ceiling faster and this is his actual top level. It's a possibility.
True.
Majority of players will develop over time.
But with some players, already at the age of 21, 22, that's it. They have reached their full potential and they won't improve anymore.
From our former players Saviola comes to my mind, an attacker who hasn't improved at all.
Actually, his first season at Barca, aged 20, was his best.
His numbers were weaker and weaker in each new season (even though he played all the time), and after Barca, he wasn't able to impose himself in any other club.
Montoya also hasn't improved too much since the age of 20.
Bartra also hasn't improved too much since his early senior days at Barca.
Rafinha hasn't improved too much since Lucho's first season. Ok, he was injured a lot, though.
Samper...
Halilovic hasn't improved at all.
He will be 23 in a Month, and after Barca, he failed in: Gijon, Hamburg, Las Palmas, AC Milan.
He is currently on loan at Standard in Belgian league.
This season, he played 59 minutes for AC Milan in the first half of a season, and 6 matches for Standard in the 2nd half of a season.
So, I would say:
1. some youngsters improve a lot over years
2. some youngsters improve only a little after the age 20-22 (Michael Owen)
3. some youngsters stagnate and don't improve at all after the age 20-22 (Balotelli, Quaresma, Anelka)
4. some youngsters actually get worse and worse over time compared to their level aged 18-22 (Halilovic and Bojan Krkic, for example)
I will stop watching the team if Barto waste any money on this Walcott 2.0