我想问一道很让我纠结的英语题~

看了这么多回答,我不觉得since I was five years old有什么问题,生活中经常用到,也常听到别人这么说 I have been doing something since I was xxx / xxx years old.
这个问题,首先要搞清楚have learned 与have studied 区别：
have learned　指学过了,强调的是结果
have studied　强调的学习这个过程
所以如果说从5岁就开始学日文了,应该说　i have studied Japanese since I was five years old. 或者I've been studying Japanese since I was five years old.
而题里这句话的意思是,我5岁的时候就学过日文了（很牛x巴）.
所以是I have learned Japanese when I was five years old.
不过,这种英文比较少见,仅仅是为了从做题出发.为了避免歧义,我们一般不这么说.