Time could slow down for him
First off, if leaping started in May 1995 and he was born Aug 1953, then he was 41 going on 42 when he leaped. As of now (2006) he'd only be 53 years old. To be 70, it would have to be 2023. But that is assuming linear time which it's possible that Sam does not experience.
Yes, time would go forward, but it is possible that inbetween leaps he is "outside time." That is, he doesn't age between leaps. It is a stasis. If you add up all the time that he was actually living in a leap (the clock ticks as he is traveling IN time during a leap), it doesn't add up to normal time. Thus, Sam may be MUCH younger than he would be had he been living on our plane of existance.
I'll say that the coma idea is intriguing and I'd like to read it. However, I would count it as an AU. I don't think that Sam is in a coma. I think he figured out a way to travel in time and that's exactly what he is doing.
BTW...Donna is older then Sam (do the math...when he arrived at StarBright in 1982, he was 28 or 29 (depending on whether before or after his birthday), Donna had just turned 30. Depending on her birthday and when he arrived, she could be almost a year older then he is. She is living IN time all the time. Thus, in 2023 she would be 71.