My rule is, if they use a cane or walker, are visibly hunched, are steadying themselves with both hands, or seem to be actively looking for a seat, I get up. Otherwise no. (The older population in Boston is pretty fit; if I offered my seat to everyone with grey hair I think I'd insult quite a few perfectly hale people.)

Also, pregnant women tend to hold their bellies. Overweight people don't. And if you accidentally offer your seat to someone who's chubby and has indigestion, well just tell them they looked like they had indigestion.

If they are noticeably older and you can handle standing there is no harm giving up your seat.