(Did I ever mention how hard it is to type when you have Shimejis?)

Happier-Tree-Friends: Whoa, Whoa, Whoa...
How Old Is Sans Anyways?!
Including You, Papyrus...
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(Before the skele-bros answer, allow me to voice my opinion :D
I've always thought Sans was older since the first time I discovered Undertale, and Toby also made sort of a side comment on Twitter about how Sans is probably the older one, so I guess it's cannon?
And about their actual ages... Well I actually don't have an opinion on that... I never really thought about it honestly...)

Now that the "Admin" is finished rambling...

SANS!!!

Sans: I'm right beside you, Pap...

I know, I'm just used to yelling your name all the time :D

Sans: Oh... Well anyways, monsters don't exactly age the same way humans do...

I don't believe we have an actual number for our age...

Sans: Yeah... But I guess you could say were in about our late teens to adult age...

That seems about right...