>>21514073When age is expressed predicatively (x is y units of time old), the units get pluralized. The girl is 20 years old, leftovers from last night's dinner are 20 hours old, etc. When age is expressed attributively (y unit of time old x), the units do not get pluralized. 20 year old girl, 20 hour old leftovers, etc. Predicative expressions are complete sentences, while attributive expressions are not. Instead, they can be labels on their own (as in OP), or they can be part of a larger sentence (The 20 year old girl has had enough time travel for one day.)
