I'm level 10 and there are differences in market value even between players of the same age
But in higher levels, I saw that the difference is no more so significant .. Like in these screenshots of players of a friend of mine at level 33 (It's not the specific players, all players in his roster are between 111 and 114 M market value)
97% 18yo = 114M
76% 25yo = 111M
Why market value gets so equal?