Chris' expected lifespan isn't dependent solely on his genes. There's a lot of other factors at play too, such as depression, other mental health factors (isn't autism linked to shorter lifespans? I thought that was brought up before but I was probably too drunk to read it), and most importantly is that he doesn't have anyone to care for him when he gets older. I've heard people say his twig limbs are a result of muscle atrophy from autism, so who knows what else that could be affecting, like his heart. He does have high blood pressure, doesn't he?
He can't rely solely on Bob and Barb's lifespans (which actually aren't even that long to begin with, if we're counting Barb being on her way out), given his lifestyle, and, more importantly, how absolutely fucked his genes are since he was born to senior citizens.
I agree genes play a big role in it, but he's really playing with fire given what his life has been.