You are not alone! I know there are some Fabbers in their 60s (at least) who like to exercise. Joy and Gaylene pop immediately to mind but I suspect there are others as well.
An idea: you might consider changing the subject heading of this thread to include your question, e.g. Workout wear for over 55? (Or something like that) You might get more responses that way.
I am in my 50s and exercise both indoors and out. You're right that even workout wear has a fashion aspect -- the fabrics and technologies change and so do the preferred (or available) silhouettes.
Looking truly fashionable in workout wear isn't really a goal of mine; I'm more interested in whether the outfit functions well for whatever I'm doing! Oh, and washes easily.
So, what I wear really depends on the season and the sport. I lift weights -- for that I wear a workout tee or sleeveless shell and workout shorts or capris. I do wear the snug ones and I don't care that I don't look like a 20 year old in them. Too darn bad! Let the viewer look away. I am there to get my exercise, not to be ogled.
For running, I wear shorts or tights, depending on season. For skating, I wear lots of layers!
I guess the main point is that I don't really factor my age into it at all. The only possible exception is for workout bras. There was a time when I seemed to be able to manage the contortions required to get those no-fasten bras on and off. But that time has past. I want a proper back fastening bra for my workouts, thank you very much.