A note for the future: his best season in his career was 2015.
He was 23 years old back then.
People here often think that the development is linear and all you need is to be young and have a lot of playing time.
Neymar hasn't improved an inch after the age of 23.
Sometimes a fast player loses pace which he had in teen years.
Sometimes a player is dumb and can't improve in a tactical area.
Sometimes it proves that a player has reached his technical ceiling long time ago.
Sometimes a player gets injured.
Sometimes off field partying or family problems ruin a player's career.
100s of things are always waiting to go wrong.
Adriano, Ronaldinho, Neymar.
Another Brasilian attacker who ruined his career due to off field reasons.
All of them peaked in early 20s.
And probably their 3 brightest talents since 2000'...
As Zlatan says: everything you do, 50% of everything you do is a mental part... (this part is often neglected on our forum):