This link provides an in-depth look at the rules for pitchers.

A basic look at the number of pitches by age:
League Age:
13-16 – 95 pitches per day
11-12 – 85 pitches per day
9-10 – 75 pitches per day
7-8 – 50 pitches per day
This is VERY basic and there are detailed exceptions and additional information listed in the link below.

Pitching Rules