Board Thread:Questions and Answers/@comment-27491885-20151228162741/@comment-31985252-20170512020522

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.