rikku was clearly more mature and developed in X2....and she wasnt a kid. what delution is there that your a child from 15 up...maybe the british are just more mature than certain other countrys (ahem).
She was a full grown woman, not a 10 year old, and she proved it by going into combat and saving the world. instead of being a background character that always worried and tried to make things lighter she stepped into the front lines, drew her blades and got dirty, the fact shes a slapper is just part of her personality like Yuna is fridgid. if anyone wasnt grown up or mature it would be paine, hiding from her feelings, unable to become emotional or reveal her true self untill at the end she was forced too. rikku had grown into what and who she was and would be. acceptance and understanding or what you are is a part of maturity.