I have every intention of staying fit, not so much for looks but because I don't ever want to have to slow down much. I love active hobbies like biking, DIY, and gardening, and it would cramp my style too much if I couldn't re-lino my bathroom or lay a stone path in my garden because I was too out of shape for heavy work.
I'll probably stop wearing such revealing clothing as I get older. After a point, it is a bit ridiculous to wear very low cut tops and very short skirts. I expect I will pack that in sometime mid-30s-ish, even if my legs are still really good. If my figure is still nice, I will continue to wear fitted clothing, especially more tailored stuff. I like tailored clothing, but feel a bit overdressed in it, generally speaking.
I'm not a trendy dresser now, I never have been, and I don't see myself ever wanting to be, so I doubt that will change much. I expect to be making my own clothing pretty much indefinitely, so however I am dressing in my 40s and 50s will depend on what clothing I like at the time and what looks good on my body by then. I have a large archive of patterns from the 1940s to the present, so I will always have a lot of options, plus I can draft my own, if I so choose.
I will totally work hats. I do presently, and I forsee myself becoming more bold with my headwear choices as I age. I may make hats my signature look, like Isabella Blow, but more sensible. Some of hers aren't even slightly attractive.