Translation:I am fifteen years old.
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
The hirgana translation of 歳 is さい? Does 才 has the same pronunciation than 歳?
the 'さい' means years so wont be used in simple counting just when referring to age.