Putting the whole married thing aside: Early twenties? gross, whatever the situation or the way he flirts. Late twenties? I suppose it could be okay. However if he’s literally constantly hitting on every single cute-ish young woman, but never with older women, it’s very problematic.

And a married man should avoid flirting unless 1- his wife is okay with it and 2 - not only he avoids hiding it, but he is very upfront about the fact that he is married.

Now if a guy such as you described exceptionally finds himself flirting with one young or very young woman in extra-ordinary circumstances, not a huge deal (as long as he doesn’t behave like a jerk).