Board Thread:Questions and Answers/@comment-27491885-20151228162741/@comment-36140716-20180710061003

I always thought Papyrus was older, and I suppose not just because of size difference. Papyrus is ambitious in the sense that he sees the importance and responsibilities of taking care of a house, having what he believes is an incredibly important job, and even feeding a pet rock.

Sans on the other hand loves playing pranks, is typically a lazy bones, but is old enough to work part time in a hot dog stand. I also assumed that Sans was the more realistic rather than ambitious younger brother, especially when he explains to Frisk that Papyrus has always wanted to catch a human, take it easy on him, etc.

However though, a counter argument to all of this is that Sans is incredibly protectively and has one hella strong attack if you've spent the game being a twat. This also insinuates that he's old enough and wise enough to judge, and to be serious and realistic as opposed to Papyrus, who's too friendly. Not to mention that TobyFox hints that Sans is older and according to Japanese translations, this theory holds true.

However, the great thing about Undertale is that few things are ever set in stone. I shall continue to pretend Sans is younger and Papyrus is older.