The Ruling on Praying in the Same Row with a Boy
Question :
Two men entered the mosque and with them was a boy, whose age was about seven or eight years old; one of them stepped forward and led the boy and the (other) man in prayer in which the man and the boy stood in a line behind the Imam. What is the ruling on their prayer, and was it correct or incorrect? And what is the age at which one may pray in the row in prayer?
Answer:
The age at which a boy must join the row is when he reaches the age of reason; this would be when he reaches the age of fifteen years, or when he has sexual dreams, or when he begins to grow rough hair on his face. But it is permissible for a boy to join the row when he reaches the age of seven years, according to the most correct of two opinions held by the scholars. And through Allah is success attained. And may peace and blessings be upon our Prophet, Muhammad and upon his family and Companions.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 2 Page 209