Hisham ibn Sa'd reported: We entered upon Mu'adh ibn Abdullah ibn Khubayb al-Juhani. He said to his wife: When (at what age) should a boy pray? She replied: Some person of us reported: The Messenger of Allah ﷺ was asked about it; he said: When a boy distinguishes right hand from the left hand, then command him to pray.
عَنْ رَسُولِ اللَّهِ ﷺ أَنَّهُ سُئِلَ عَنْ ذَلِكَ فَقَالَ إِذَا عَرَفَ يَمِينَهُ مِنْ شِمَالِهِ فَمُرُوهُ بِالصَّلاَةِ