Vegeta would answer that but... has to consider again. "...I'm not sure what the average age span of a human is." Look, he would guess it's around 80, that seems about right from what he's seen, but then you got random beings like Roshi, so who knows what the typical human lived to. It's also the factor in that Vegeta hasn't cared enough to find out in the past. Having such an elderly population on Earth had actually taken some time to get used to, he wasn't used to seeing so many.