"They" are probably the same twee folk who warn us wearing white shoes after Labor Day is gauche. And that legs should be panty-hosed in offices, sheathed like sausages, year-round. "They" aren't living around here, that's for sure...where it's 100 degrees, muggy and buggy.
My grandmother, who at 80-something still goes to Silver Sneakers aerobics at the local Y three times a week, says a woman knows she'd old when she waves to someone and her upper arms keep waving and waving....
What can you do? Laugh. Laugh and just accept everything is temporary, even your toned upper arms. Unless you're a vampire.
I admire Michelle Obama's bare arms--
But Madonna

After a while, it's just easier to observe and watch life--passing you by or not, flabbing up your arms maybe, but bringing you wisdom. Hopefully.