It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he (peace and blessings be upon him) said: "The pen has been lifted from three people: from the one who is asleep until he wakes up, from the child until he reaches puberty, and from the insane until he regains his reason."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5089
Sayyidah Aishah (may Allah be pleased with her) says that the Prophet (peace and blessings be upon him) said: "Allah Almighty does not accept the prayer of a girl who has reached maturity except with a head covering."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5090
Abd al-Malik bin al-Rabi' bin Sabrah narrates from his father, and he from his grandfather (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said: "Teach children the prayer when they are seven years old, and at the age of ten make them pray, even by smacking them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5091