"他"指的是谁？你想问的是关于某个人什么时候开始打球的问题吗？

如果是这样的话，你可以这样问：

什么时候开始打球？
你什么时候开始打球？

这个问题的翻译可以这样写：

At what age did he start playing sports?
When did you start playing sports?

下面是一些英文例句：

He started playing basketball when he was in middle school.
She began playing soccer when she was five years old.
He started playing tennis when he was a teenager.

这些例句的中文翻译分别是：

他中学时就开始打篮球了。
她五岁就开始踢足球了。
他十几岁时就开始打网球了。