The reaching puberty of a boy is by way of nocturnal emission (i.e. a wet
dream), ejaculation, or by his causing pregnancy when he has sexual
intercourse.
If [any of] that is not found, then [he is a minor] until he has completed
eighteen years [of age], according to Abū Ḥanīfah, may Allah have mercy on
him.
The reaching puberty of a girl is by way of menstruation, nocturnal
emission, or pregnancy.
If [none of] that exists, then [she is a minor] until she has completed
seventeen years [of age].
Abū Yūsuf and Muḥammad, may Allah have mercy on them, said that
when the boy and the girl have completed fifteen years of age then they have
attained puberty.
When a boy or girl approach puberty, and their position concerning
majority is difficult [to ascertain], and they say that they have attained
majority, then their statement [is accepted] and the judgements [on them] are
the judgements on mature people. - Fiqh Quduri 63-2-1 (islam)