Undertale - So, uh, a lot of you wanted to know how old we are. Not sure how it translates to human years, but there are some pictures on the fridge if you want to check ‘em out.

I don't think it's confirmed one way or another, but it's a funny way to explain why Pap is so childish; in this context, he's still a kid himself.

Sans hasn't changed that much all those years.