Choosing the right basketball is very straightforward once you know the age group and gender you are looking for, as sizing is determined by both of these factors. 

The Size 7 ball is the men's ball that is used from Under 15 upwards. 

The Size 6 ball is used in the female game from Under 13 upwards, and is also used for boys aged Under 13 and Under 14. 

Size 5 balls are used for both boys and girls at primary school level up until Under 13.