MK2 was 17 and up (the same as "Mature")-- at least that's how it was on the Genesis and the various 32-bit versions. The Nintendo rating might've been a little weird since they pretty much had to create a rating system just for that game, but the one Sega used had been in place for quite a while before that. In fact, the only MK game I know of that wasn't 17 and up was the Sega versions of MK1, which were 13 and up (excluding the Sega CD version), since you needed a code to put the blood in.
Of course, these are the ratings for the US versions.