My 5 year old son has been asking for fish for over a year. He went so far as to take out every book on fish in the library, plan out what kind of fish he wanted, and was able to tell me exactly what I needed to buy to make it all work. He's a bit of a self-motivated kid. My husband and I had said we'd get him a tank for Christmas, but then summer camp came around and one day he walked out with a minnow, a snail and a piece of anacharis in a 1/4 gallon container. The minnow didn't live long, but the sheer joy in his face when he sat talking to the fish was infectious. We bought a 10 gallon tank and the guppies he wanted. And then came the 20 gal setup. And the nano snail bowl.
I suspect that's just the beginning.