ESRB looks onto certain aspects of a game, to see if it fits with their standards. We can't just relay on them and their judgements can be sometimes incorrect.
I don't think this game shouldn't be played by youngsters. I would rather say this game was meant to be created for people from age 9-99

- it has everything that would attract both kids and mature people. It's obvious that there are some things, that might be perceived as not suitable for youngest kids, but it merely bases on things like the ingame atmosphere or simple elements, which can be treated as nasty or horrible (the tissues in cathedral, coating all the walls and things can be seen as really disgusting), but it mostly depends on a kid, which is playing the game. Some might be afraid, some will be rather amused. It's a fact that small kids won't take much from the game and they won't understand everything, but this is not the thing we're talking about. Small kids are playing games mostly for fun, so we shouldn't be concerned about this - only the older ones will start focusing on story a bit depper, and they will propably ask us about things that might amused them

. It's normal way, as for kids.
If I'd had a kid, I would firmly put him besides the pc and turn Aquaria for him. Furthermore, I would play the game with him, so it could understand everything. In my opinion Aquaria has a beautiful story and it brings many morals and ideals. It can teach many things, especially the children. When I finished the game, I realized once again that the true substance of our existence lies within us and above all, love is one of the most important things in our life, although we still forget about it, lost in disorder of our everyday lifes...