I dress trendier now, in my 60s, than I have dressed since my mid-20s. Between my mid-20s and around the age of 50, I didn't have a lot of disposable income, so I wore whatever jeans and tops I could find at the local Winners. They were rarely on trend.
I don't think at all about age appropriateness. I do think about practicality, as Brooklyn mentioned; about my own preferences (do i like this trend? If not -- I don't think about it); and I think about which parts of the body I may not want to exhibit to the air.
Some of that is constant. For instance, I've almost never worn mini skirts. I've always been self-conscious about my thighs, and I feel exposed in a short skirt. But I will wear shorts when I need to (garden, bike, etc. ) and don't care what others think then. So practicality overcomes other considerations.
But -- to give a different example -- I might have worn a midriff baring top as a teen or 20 something, and would have worn a tube top or a skimpy cami. I wouldn't do that now, not out of fears about my age, or flattery considerations, exactly -- more out of a feeling that I don't want to put my skin so much on view. Increased modesty, I guess.