The average age of husband and his wife was 24 years at the beginning of their marriage. After 4  years they have a one year old child. The average age of the family of three, when the child was born was:

18 years

At the beginning of their marriage ,

The average age of a husband and his wife = 24 years

Total age of husband and wife is = 48 years 

After four years of marriage,

Age of ( Husband + wife + child ) = 48 + 8 + 1 = 57 years

At the time of birth of child, age of ( Husband + wife + child ) = 57 – 3 = 54 years

∴ Required average age = \(\frac{54}{3}\) = 18 years