看了這么多回答,我不覺得since I was five years old有什么問題,生活中經(jīng)常用到,也常聽到別人這么說 I have been doing something since I was xxx / xxx years old.
這個問題,首先要搞清楚have learned 與have studied 區(qū)別：
have learned　指學(xué)過了,強調(diào)的是結(jié)果
have studied　強調(diào)的學(xué)習(xí)這個過程
所以如果說從5歲就開始學(xué)日文了,應(yīng)該說　i have studied Japanese since I was five years old. 或者I've been studying Japanese since I was five years old.
而題里這句話的意思是,我5歲的時候就學(xué)過日文了（很牛x巴）.
所以是I have learned Japanese when I was five years old.
不過,這種英文比較少見,僅僅是為了從做題出發(fā).為了避免歧義,我們一般不這么說.