I'd say they are about 12 or 13, mostly because when fillies get their cutie marks they have something called a cutesinera which seems equivalent to a quensinera (I probably spelled that wrong, oh well) I agree with the whole cutie-marks-being-puberty-idea. Besides, in EG the CMC were in the school, I heard that the school they were in (Canterlot High?) was a middle/highschool combination. So they are probably 12 or 13, in 7th or 8th grade. (That took a while . I feel smart write long words that a probably spelled wrong!)