We never say 'fifty years old man' we say 'fifty year old man' but we do say 'he is fifty years old' 'five dollar bill' not 'five dollars bill'

Fifty years old man

"Fifty year old man".

Fifty-dollar bill, fifty-year old man, fifty-minute game fifty buckets of ice cream, fifty pieces of gum, fifty weeks in April, fifty cents That is a fifty-dollar bill, three half-dollars, fifty-fourth cup of coffee

fifty-year-old man and five-dollar bill.