Mozart started writing music when he was four years old.

问题描述:

Mozart started writing music when he was four years old.
以上是一个when引导的时间状语从句,请问这个状语从句修饰主句中的哪个词?
麻烦分析一下句子结构?

状语从句是来修饰谓语动词的也就是when he was four years old,来修饰谓语动词started的,