Babybones papyrus and adult(ish)!Sans AU's are all fun in games untill papyrus grows up.
It's all cute untill you realize the gap.
Sometimes its hard to tell whether sans is an adult or just some teenage kid. (Probably cause he's short and stylizagion and characterization don't change much in most iterarions.)
Like, genuinely??? If sans was old enough to work a job as a scientist (and have some sort of degree), I'd expect him to be atleast a young adult or full grown at that point. (If he can go to a bar, play poker and work as a scientist? He's old enough to be a dad. Hence, old enough to be a grandpa.)
So when babybones papyrus becomes adultskeleton Papyrus, sans should either be a full grown adult or a grandpa, or someone in their mid-30s to 40's life crisis.
He's an old fart.
[EDIT: only boss monsters age after having a child, that was my bad]
And since monsters only start to naturally die after having a child, sans is eligible to become an thousand year old wizard.
Which in turn questions the fact of how that actually managed to happen. Using human fertility expectancy as a guide line, it is safe to assume that if they are related in anyway that closely links them as siblings.. it means who ever their parents are, are probably old enough to be grandparents. If they're still alive by the time Papyrus is a full adult (not to mention if he even gets to elderly, or lives to see the end of time itself-) they could be great grandparents.
Of course, this would be (almost) unnoticeable, as skeletons don't have skin that immediately indicate age. Since these are monster skeletons, their growth and appearance is quite diffrent from humans, thus cannot be compared exactly. Though if one is to assume that they function identically, then examining the size, growth of certain featutes, the bone itself (like marrow and decay) as well as its health, might aid in determining age accordingly. That is of course, if features such as obvious canes, bone problems, and other skeleton mishaps that occur in elderly are showcased publicly.
To avoid this, one might create a distance in relation by having them be adopted, cousins, nephews or uncles, or not have them related at all and be compelte strangers to one another.
Either that, or they take the high road and have both skeletons be near of age (12 and 3 or 4 and 6. Maye even have an age gap equivalent to that of twins- a gap which is measured by a matter of minutes and seconds.) To avoid further confusion.
Otherwise, if sans or papyrus are old enough to go to a bar, get a high degree job, and are fully fledged or young adults when the younger sibling is still capable of being considered a "baby" ,it can be assume one may grow to be 40 whilst another would still be 16, 18. A considerable age gap.
Perhaps monsters fertility works diffrenetly than humans, but it isn't stated as such in the game as far as I'm aware. As of right now, Adult(ish) sans and babybones papyrus would result in a very large age gap, where sans could be a dad, or even a grandpa if he's old enough, whilst papyrus could still be in what? Colledge?
