if he is just starting then 250 beg
prefferably the 85cc class
125 or 250 if you fit on the but but they run together in the 250 class
25+ vet A and vet B or 40+
They would have to be in a 90 cc class or lower to race quads. But you can be any age to race dirt bikes.
85 cc or 150 cc
Depends on height and weight.
125, 250, 450
probaly like 15 years old or more you just have to the right size to fit it
a 250cc
30 is considered an old timer in professional motocross. There are a few exceptions though, as long as you stay healthy (injury free) and are competitive.
It depends upon each countries specific ruling but generally 13 years of age
4 years old you can race motox. (50cc class)