How old was Erza scarlet when she became magician class S ? any one can answer?

15..It Is Stated In The mangá !!

Hmmm, well if I may say she was the youngest Fairy Tail wizard to become S-Class and por the picture I saw she looked like she was 12 or 13 years old at the time.

17

Yes it's 15. And for Mira it's 16.