More to the point, there is no law against finding a 15 year old attractive, provided you aren't doing anything illegal based off that attraction. There are 20 year olds who look like 15 year olds and vice versa in this world. Whether or not Rikku is someone you find attractive is up to you, of course (I don't find her unattractive or attractive, personally, she just seems like a generally cute person), but I'm not gonna judge.
FFIX wins this battle for me but I thought it would be closer than the polls are saying right now. Still, there is time.