Board Thread:General Discussion v2.0/@comment-24833746-20151116053145/@comment-4690133-20160119021933

Fleebe wrote: Hell and damn are not really swear words. I'd say E10+. I'd say it'd be E10 for all that. Hell and damn are swear words according to the ESRB.
 * Mild Blood 'Reference'(after fighting sans, he does not really bleed, it's most likely ketchup)
 * Violence(It's everywhere)
 * Crude Humor(at some points)
 * Drug References(Grillby's and Burgerpants)

EarthBound and it's prequel were rated T. No way in hell Undertale wouldn't.