For adults, the UL for magnesium is 350 mg per day. For children ages 4 to 8 years old, the UL decreases to 110 mg per day, and the UL for children 1 to 3 years old is 65 mg per day.agnesium oxide is a common salt formulation of the mineral magnesium. Your body needs magnesium for numerous vital biological functions, including maintaining a steady heartbeat and proper nervous system function.