How old is she?

This past week, we've had a lot of people making the same basic mistake:

1. "a 7 years old girl"
2. "a 5 years boy"

In the first, "7 years old" looks like a noun phrase, but it is functioning as an adjective. It modifies "girl." In cases like this, "years" should be singular, not plural, so "a 7 year old girl" is correct.

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."