[Abu 'Eisa said:] This Hadith is Hasan Sahih. This is acted upon according to some of the people of knowledge. They consider pubic hair an indication of the age of responsibility, if it is not known whether he has had a wet dream, or his age. This is the view of Ahmad and Ishaq.
In-book : Book 21, Hadith 46
English translation : Vol. 3, Book 19, Hadith 1584