Generally, the average age of a ball boy in tennis is 15 years, and the maximum age limit is 17 years.
Here is the list of ages for each tournament.
Hobart International: 10 - 17
Australian Open: 12 - 17
Adelaide International: 12 - 17
ATP Cup, Brisbane: 12 - 16
ATP Cup, Sydney:  12 - 16
ATP Cup, Perth: 12 - 16