Reagan was 69 when he was elected I remember the left making a hell of a big deal about his age back then. As for the current crop some people age better than others It seems like Hillary is the one who has had the most health issues out of the three listed in the OP.
Reagan exercised a lot. Hillary gets no exercise and she is unhealthily fat. Trump is fat, but not so bad as Hillary. But Trump also gets no exercise. Sanders is skinny. He looks like he exercises moderately.
Hillary has had several mini strokes. She gets confused and needs several naps day. (as attested by the emails on her server.)She suffers from early Alzheimers
So of the three, Sanders is healthy, and might make 90. Trump would not make it through four years . Who he would chose as VEEP would be very important. Hillary is in this year's Dead pool, and I would be surprised if she makes it to August.