Overall I'd also say that the bosses were too easy, like in all other 3D installments, I think they just haven't found the right way to make 3D bosses a bit more challenging. If you take away Z targeting, then things will be too hard, with it it's too easy, that's a problem they didn't have to deal with in the 2D era. Hopefully, with the Wii Motion Plus it will be a bit harder.
And I also don't think that it depends that much on age. Granted, I was 20 on my first OoT playthrough and found the bosses quite easy, but I was 7 when I played aLttP for the first time and even now after many playthroughs it hasn't become that much easier, for example sometimes I still fall down a floor against Moldorm several times per fight.
And I also don't like the difficulty development as you progress in the game in OoT. Compared to other bosses in the game (adult part), Phantom Ganon was probably the hardest one (1), Volvagia wasn't too easy either (2), but then Morpha (5) - a total pushover! Bongo Bongo (3) was a bit tougher again and Twinrova (4) - very easy again. The fact that your heart bar usually grows during that progression adds to that strange development. And I didn't like how Ganon was much easier in beast form (roll/slash, roll/slash, ...) than he was in the first battle, especially if you suck at timing and quick aiming like I do.