If you passed a fit young person coming into a busy venue as you were exiting, and that person wasn't wearing a mask, would you say anything to them?
I wouldn't because you've got no way of knowing if that person is exempt. It is also possible the person does have a mask and had simply forgotten to slip it on. I think they will pretty soon notice others wearing masks and it will remind them if that's the case.
I've witnessed other people making comments to non mask wearers when in public though.