Well besides NPCs telling you to go fuck yourself or to die close to the underground shelter so they can loot your corpse, anyway. The maturity is required to understand the concepts the game is trying to press. A young player wouldn't understand things like gods of fertility, what the diet building is, or nuclear missiles. They wouldn't be able to relate to the more adult concerns expressed in Shin Megami Tensei. While they might not have any problem playing the game itself, they certainly wouldn't be able to appreciate it as much as adult players would. Like a childrens show making adult humor, these things will just go in one ear and out the other.