How to play this spot with AJ is player-dependent. If you think you're ahead, you should usually raise, and if you think you're behind, you should usually fold, but the bet size and position alone don't give us very much information.
What kind of player is SB? Does he bet out of position like that often or rarely? Do you even have a general impression of how he plays or how he thinks?
If you have no info to work with, just play it safe and fold in this spot. Don't put a big chunk of your stack in jeopardy in a limped pot with this mediocre hand.