fifty · number /ˈfɪf.ti/Full view
A2 the number 50
Dictionary examples:

forty, fifty, sixty

They were travelling at fifty miles an hour.

Learner example:

There were fifty guests at my party. (Key English Test; A2; Russian)

(be) in your fifties
B2 to be aged between 50 and 59
Dictionary example:

He retired when he was still in his fifties.

Learner example:

Despite being in my fifties I am healthy and in good shape. (First Certificate in English; B2; Polish)

the fifties
B2 the years from 1950-1959
Dictionary example:

Many new towns were built in the fifties.

Learner example:

I love bands from the fifties, sixties, seventies... and most of the radio stations seem to forget that the music has a history and it is always nice to remember past songs, because a good song will always be a good song. (First Certificate in English; B2; Spanish)

Cambridge University Press