Average age of 4 family members of a family is 15 years. The average age of that family before the birth of their 3 years old twin kids was:

24 years

Average age of 4 members = 15 years

Sum of age of 4 members = 15 × 4 = 60 years

3 years ago sum of age of family (having two person only now) = 60 - (3 × 4)* = 48 years

Average age of family (3 years back) = \(\frac{48}{2}\) = 24 years

*Note: To find the age of 3 years ago: the age of two child i.e (3 + 3) = 6 years and also (3 + 3) = 6 years of two other family members will also be deducted. Therefore, total 12 years deducted here.