Bambina is from first months until more or less 10 or 12, it can't be said
precisely as there are big individual differences at that age.
/Ragazza/ is what the English call -teen age, that is ending around 20 22;
at that age usually big changes in the brain usually happen that will (partly!)
make both males and females think (much more) in advance to what they do;
it is actually a 'turning point' due to physiology, although not so evident as the one that make the transition from bambino/bambina to ragazzo/ragazza, which is due to hormones and affects so much of the body and of the behaviour.
I think of my students at University classes (especially after first year) as young men and young women.
Due to sociological reasons, you will often hear the terms /ragazzo/, /ragazza/ given to people well after twenty years old in Italy, but that is just a BIG LIE in front of Mother Nature!