It says goldfish reach maturity at age ten, i.e they stop growing. For all we know, she could be a lot older.
The story seems to suggest she's actually 10 or just a bit older, but from a goldfish perspective she's the equivalent of 18 or 20 in terms of maturity. I think the suggestion is in most ways the two characters are equals, save for physical appearance.
Using this as my excuse for liking it so I don't feel too disgusted with myself.
I'm disgusted with everyone who can't express his / her liking towards something directly, but tries to find excuses for that instead.
I am disgusted with everyone who is disgusted with others... wait...
Gotta say, it was getting quite charming... Until the threesome (or just when the 3rd character was introduced) happened, kinda broke the charm I was getting from this. Even so it is OK I gues.