Move people have a natural tendency to unthinkingly assume anyone they interact with as equals to be "about my age" - works in both directions. In your 70 year old neighbor's mind, he or she may well see him or herself as looking not that much older than you!
Lots of people (not just elderly ones) have such a strong self image that it isn't remotely shaken by what they see in the mirror :o and they can see the objective fact that they have an eyelash on their cheek or food between their teeth and deal with it, without their self image being remotely shaken by the fact that the objective person in the mirror doesn't fit with their idea of themselves, who they are and how they look.
So many people think of themselves as ugly despite being average looking or attractive and vice versa, as looking young when they no longer do, as looking haggered when they look pretty fresh and well preserved, etc etc.
I expect most of us have met someone who thinks they look 10 or 15 years younger than they do and fishes for compliments about it when they really, really don't... Most of us go along with it too, because there's nothing to be gained from trying to point out the delusion and they make it blindingly obvious how they want to be complimented (If you didn't know I had a 20 year old son, how old would you think I was? If you didn't know I've been working here for 20 years, how old would you think I was?" type questions are blatantly fishing for "Oh you don't look old enough to have a grown up child/ a long work history, you look about 30! "etc)