Board Thread:Questions and Answers/@comment-27491885-20151228162741/@comment-27123315-20180320023811

I think of them as twins, Papyrus is the physically stronger one (more hp, taller) and does most of the chores while sans pays the bills, works, and emotionally supports his brother (helping to make his "armor", and being "santa".)

They've always seemed like two people who have both known each other their entire lives based on their chemistry, and grew up together, so even if one is older, I don't think it is a substantal amount of time.