I teach kindy to grade 3 as well as adults ... I love the little 1-3 year olds because they try so hard and are always out to please the teacher... they thrive on attention and compliments and are so cute !
I enjoy the grades 1-3 as well, but enjoy the fact that we can work on more complex tasks, ongoing projects, web searches etc... and the kids are more independent. With this group, you have to work a little harder to keep the students interested and to gain respect; but the result (if successful) is a more sincere respect from the kids, rather than a blind obedience that the younger kids give.
With adult classes, I like the occasional complete beginner, although this level can be very draining if taught in excess !
I liketo space the beginner adults between a couple of more abled adults, who allow for more conversation and discussion to keep the brain active.