"十五さいです。"

Translation: I am fifteen years old.

[Andrew-Lin]

十五歳です。(formal) or 十五才です。(common because easy to write).

[googols]

天才(Genius) lol! Sorry for my bad English.
Also 才 is used by Japanese children 'cause almost Japanese children do not learn 歳 until in middle school. (Japanese children learn 才 when they are 2 Grader)

Source in Japanese:http://zatsugakuki.blog78.fc2.com/blog-entry-230.html

[344bc028-46b5-4f]

The hirgana translation of 歳 is さい?
Does 才 has the same pronunciation than 歳?

[JeffMather]

Yes.

[Max49576]

the 'さい' means years so wont be used in simple counting just when referring to age.

[Lily_Trujillo]

I accidentally wrote 14, but it was just marked a typo, luckily.