Hazrat Musleh Mau`ud (ra) has said:
In my opinion, the command for fasting is from 15-18 years of age because this is the age of puberty or maturity. One should practice at the age of 15 and make it mandatory at the age of 18. When we were kids, we used to want to fast, but the Promised Messiahas would stop us. For a child’s health and strength you should stop them from fasting. When they are 15 they should slowly begin fasting and increase gradually.
(Al-Fazl, 11th April 1925)
Spiritual Fitness 