A word or expression for an older man trying to look younger

Solution 1:

Putting aside the odious ageist assumption that doing one's best to keep fit amounts to "trying to look younger" (suppose they want to feel good and live longer?), and since you are supposedly "not looking for a derogatory term but possibly a politically correct one," why not simply use the same kinds of language you would use for other age brackets:

fashion-forward seniors

fashionable older men

men in their golden years and dressing well

older men who take good care of themselves

hip baby boomers

etc.

By the same token, if you did want to offend/denigrate, you could use the same strategy

retirement-age chavs

'bros' pushing 70

elderly metrosexuals

etc.

Solution 2:

There is a term mutton dressed as lamb but mostly applied to old women. It is also used for old men sometimes.

(idiomatic, derogatory) A mature woman dressed in a style more suited to a young woman, especially if in a deliberate attempt to appear young.

For men, mutton dressed as ram version is used also.

There is a related term medallion man also, though they have a specific style:

a man who tries to look younger than he is by wearing jewellery and tight clothes, and who often has his shirt open at the front to show his chest

As a formal term, you can use well-groomed or soigné.

Solution 3:

"Those dapper older gentlemen..."