Alot of it depends on your and the horse your ride. At the lesson barn where I ride, there is a student who is physical ready to canter, meaning she has excellent balance and rythm, but mentally, she is afraid to canter,
So alot of it is your progress, ask your trainer to tell you how you are doing, and if you feel completely confiedent at your skills at the trot, and your confident in the horse, go ahead and ask your trainer to teach you to canter! And its okay if it goes completely wrong. The first time I cantered, I didn't steer the pony and jumped poles at the same time!!!! But it will be your most memerable moment!