What a fabulous question, Peggy, for all of the teachers 'out there'. I was a dance teacher for several years and loved all of my students no matter how old. I taught from ages 2 through 70.
My adult students were serious about class and practice, despite all their aches and pains. The teens were a riot because their bodies were so gorgeous and flexible but they were so into their issues.
Then there was the toddlers with their little tutus on...what a riot! I think that ages 2-6 would be my favorite age for learning. They are curious little sponges that find the world a magnificent place to be.