After seeing a True Pacifist Playthrough (did not see any other routes) I checked out the Undertale wikia. During the playthrough I noticed Sans and Papyrus talking about who's older. If I recall this correctly, there was something along the lines of 'Papyrus, but Sans reads night stories for him therefore Sans could be his older brother'. I don't know whether there were any other hints.
I looked at the Undertale wikia page for Sans and found that Papyrus is a younger brother to him. (Link)
Is there any other information that hints at who is the older brother?
Best Answer
There's no definitive in-game/canonical evidence as to which brother is older.
It's obvious that they both care about each other, however the Wiki entries for Sans and Papyrus just list them as 'brothers', and everything in-game pointing one of them being the older sibling is circumstantial at best - most likely played this way so as to leave it up to the player's own interpretation/head-canon.
Some parts of the story points to Sans being younger:
Other pieces of the story point to Sans being older:
There are some other fan-theories & explanations out there, for example, that they are as old as the creation dates of the fonts the characters are based on: Comic Sans and Papyrus, but again, as these are just fan-theories and aren't confirmed in-game in any way.
Finally, it's worth mentioning that the developer of the game - Toby Fox - has also previously tweeted that Papyrus might use the Japanese word 'Aniki' (i.e. 'Older Brother' or 'Superior' in Japanese):
Again, this isn't covered in-game and Toby has since deleted this statement. Thus it shouldn't be interpreted as 'Word of God' (TV Tropes). Even if the statement hadn't been recanted, there is still enough evidence to point either way in game, thus it would still be open to interpretation.
Until (and if) we get another source of official canon material (such as a second game, comic, movie etc) that builds upon or embellishes the story, we don't have the evidence to say which brother is older than the other. It's ultimately up to you to decide.