Board Thread:Questions and Answers/@comment-27491885-20151228162741/@comment-31750551-20170626205136

Sans is the older brother. There is a lot of evidence, but I'm too lazy to write it here so just read the earlier posts by other people.

Meowkid1000 wrote:

2Lazy4Username wrote: I think that Sans is the older brother. He is stronger than Papyrus and his battle(genocide route) is said to be the hardest boss fight in Undertale. He also kind of acts more mature than his brother and lastly, when he is talking(text), there's always that low-pitched sound that you hear. These are the reasons why I think that Sans is older than Papyrus. this is all very true, maybe besides the fact that he only has one hp. It's never said that he has 1 hp. Unless you're talking about the game files, which say that Mettaton NEO is the strongest character and Asriel is weak.