i began to study English as early as 5 years old.

1.这句话是对的,5 years old 是一个短语 当然可以跟在as ……as后面.
用as early as 是一种强调,表示 我很早就开始学英语了,
I began to study English when I was five years old.意思一样,但是语气不一样,没有as as的语气强烈.
2.如果你觉得有疑问,用I began to study English when I was five years old.足够表达你的意思了,只是语气上有欠缺.