An idiom for a young-looking older person

an old/a wise head on young shoulders.

1. What is the idiom for looking young?
2. What is the idiom of age old?

What is the idiom for looking young?

Mutton Dressed as Lamb

This idiom means that somebody is dressing like a much younger person. For example: The style doesn't suit her, she looks like a mutton dressed as lamb.

What is the idiom of age old?

age-old | Intermediate English

very old, or having existed for a long time: It's an age-old story of love and betrayal.