In 星の王子さま, what is this 六つのとき?


In the first page, this is written:

>
> 六{むっ}つのとき、原始林{げんしりん}のことを書{か}いた「ほんとうにあった話{はなし}」という、本{ほん}の中{なか}で、すばらしい絵{え}を見{み}たことがあります。

I don't have the English version, but the Brazilian version says the
equivalent of:

> Once upon a time, when I was 6 years old, I saw in a book about the Virgin
> Forest, Real Stories, a impressive drawing.

Would it be correct if I said something like 私が六歳のとき?




> (私が) [六歳]{ろくさい}のとき...  
>  (私が) [六]{むっ}つのとき...

Both are correct.

「～歳」 can be used for all ages, and 「～つ」, for ages from 1 to 9, as in
「ひとつ、ふたつ、みっつ、よっつ、いつつ、むっつ、ななつ、やっつ、ここのつ」, and 「とお」 for 10. 「～つ」 is more
casual/informal, and can sound a bit more childish than 「～歳」 (We use 「お幾つですか？」
for adults, too, though).

> Adult (toward a little boy): 「ぼく、[幾]{いく}つ？」/ 「ぼく、[何歳]{なんさい}？」  
>  Child: 「[３]{みっ}つ。」/「[３才]{さんさい}。」

