At what age did you start to become interested in xx?
How old were you when you first took an interest in xx?
この場合、次のような聞き方ができますよ。
ーAt what age did you start to become interested in golf?
「何歳でゴルフに興味を持ち始めたの？」
At what age did you start to become interested in xx? を使った聞き方。
ーHow old were you when you first took an interest in medicine?
「何歳で医学に初めて興味を持ち始めたの？」
How old were you when you first took an interest in xx? を使った聞き方。
ご参考まで！
カナダ 
