Question

【题目】I started riding a bike when I was ten years old.
I started riding a bike ten.

Answer 1

【答案】at；the；age；of
【解析】句意：我10岁的时候开始骑自行车。when I was ten years old我10岁的时候；表达年龄的短语是：at the age of，在……岁时，所以答案是at the age of。