4 years.
X + 3X + 6X + 9X + 12X = 50 5X + 30 = 50 5X = 50 - 30 = 20 X = 20/ 5 = 4
4 years. Due to the regular intervals, the average age of the children is the age of the middle child = 50/5 = 10 years. The youngest child will be 6 years younger = 4 years.