- "a 7 years old girl"
- "a 5 years boy"
The second has the same mistake plus an additional mistake. Since we are talking about age, we need to add "old," so it should be "a 5 year old boy."
However, if we're not talking about age, then "old" is incorrect. So when a professional athlete signs a new contract, then we say he signed "a 5 year contract."