I am not into the legging trend, but I think someone in their 40’s could easily wear them and look just fine. A lot of fashion has more to do with someone’s body type, their figure, than age. Well, I would say both have importance.
Like low rise jeans for instance. If someone is thin, they can wear them easily until age 60 (not the super low rise see your butt crack even when standing straight, no one should wear those) but even a 20 year old that has a huge fat roll over top of the waist of the jean needs a new fashion. At the same time someone who is 50, in great shape, can wear a halter style top and short shorts and look great, but I think a lot of people might think a 50 year old can’t or shouldn’t.
Fashion is also very regional. Depending on what part of the country, fashion might be more conservative than others. Some cities it is odd to let a bra strap show when wearing a tank top, other parts of the country no big deal, even trendy.
I see nothing wrong with men wearing shirts that stay stuff. Were you also including shirts that say a brand name? Like Porsche? Or, Harley Davidson? Any age can wear those. I do object to swear words or crude sayngs on shirts for any age.