Soccer balls used in youth leagues come in sizes 3, 4, and 5.
3-7 Years Old (U4-U8): Size 3
8-12 Years Old (U9-U12): Size 4
13 on up (U13-U18): Size 5
A Size 5 ball is the same used by adults. There are also Size 1 balls, which are often called 'skills balls' that players can use to practice foot skills.