Is he 48 now? So he'd be 50 or more? so in his 70s when the child was 20? likely to be in his 80s when the child married/settled down/had kids of his own?
Really depends on the individual. Some people are old at that age, really old. Nursing home old. Others are still working!
What are his parents like? What about his grandparents?
I know it's not scientific or anything
just my observation, but I have noticed that if someone is old by a particular age, their parents, grandparents, etc, often were too. I'm not saying this is everyone. I am just telling you my personal observations, I don't want my arse on a plate with a pitchfork garnish 
My husband, for example, is nearly 50 but looks like a much younger man. His mum is in her 80s and still farms! Walks several miles a day. Looks 60.
My parents, otoh, aren't even 60 yet and they're old!