(a) Imam al-Shafi'i (may Allah have mercy on him) said: In Sana'a I saw a grandmother who was twenty-one years old. She began menstruating at the age of nine and gave birth at the age of ten, and her daughter began menstruating at the age of nine and gave birth to a child at the age of ten.
(b) Hasan bin Salih says: I knew a neighbour of ours who became a grandmother at the age of twenty-one.
(c) Mughirah al-Dabbi says: When I first had a wet dream, I was twelve years old.
(d) It is narrated from Sayyidah Aishah (may Allah be pleased with her): When a girl reaches nine years, she is a woman; that is, (if) she menstruates, then she is a woman. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1531