The average age of husband and his wife was 23 years at the beginning of their marriage. After five 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 = 23 years

total age of husband and wife is = 46 years 

After five years of marriage,

age of ( Husband + wife + child ) = 46 + 10 + 1 = 57 years

At the time of birth of child, age of ( Husband + wife + child ) = 57 – 3 = 54 years
∴ Required average age = 54 ÷ 3 = 18 years