This game was sooooo much fun a few years back (7-11). Trust me, if you're not playing with at least one other person, you're doing it wrong. It is much more competitive/cooperative to work together to beat a boss or throw your friend off a cliff while you grab the chest you agreed to let him have.
Here's something. Me and my cousin, who where respectively 9 and 5 at the time, beat this game. When we were 11 and 7, we managed to get the Gold Keys (3,000+ rupees). When we were 12 and 8, we got most of the Hero's Keys (6,000+ rupees). Most, because we never got around to the last one. Didn't have the time.
Man, the Hero's Keys are hard, they require total cooperation. You have to be in-sync with your partner(s). The only way we found to get enough was to have rupee fever, which doubles the amount of rupees you earn when all players have full hearts, making black ones worthwhile! For some reason, there is no rupee fever in the single player version of this game, which seems to make it near impossible.
So for my final statement: PLAY THIS GAME, AND BE DAMN SURE TO FIND A BUDDY. Make sure he is a total jackass when playing (like some friends of mine), or can be totally in-sync with you so as to get those Hero's Keys (like me and my cousin.)