I say, and with God's success: The prayer of a mature person is not valid behind a non-mature person. Usually, a male reaches maturity after twelve years, so a mother’s prayer behind her son is not valid if he is not mature. It is an exception to allow leading prayer for someone who has reached ten years if he has memorized the Quran in voluntary prayers like Taraweeh and not otherwise. This is in the opinion of the correct scholars, and God knows best.