فيجب الغسل … على الفاعل والمفعول به إلا إذا كان الفاعل أو المفعول به صبياً أو صبية فإنه لا يقال وجب عليه لأنه ليس مكلفاً ولكن يقال صار جنباً فإن كان مميزاً وجب على الولي أن يأمره بالغسل كما يأمره بالوضوء
المنهاج في شرح صحيح مسلم للنووي كتاب الحيض باب بيان أن الجماع كان في أول الاسلام لا يوجب الغسل إلا أن ينزل المني وبيان نسخه وأن الغسل يجب بالجماع
Washing is obligatory … for the one who does it and the one who gets it done to them, except if the one who does it or the one who gets it done to them is a young boy [sabiy] or young girl [sabiyya], in which case it is not said to be obligatory, since a little boy is not yet responsible for his actions, although it can be said that he has become sexually impure, and if he is of the age of discernment it is obligatory for his guardian to ask him to wash, just as he asks him to perform ablution.
Al-Nawawi, Al-minhaj fi Sharh Sahih Muslim [Open Path in Exposition of Sahih Muslim], The book of menstruation, Section: substantiating that in sex at the beginning of Islam, washing [ghusl] was not obligatory unless semen was discharged, and substantiating that this was abrogated and that washing is always obligatory from sex