As you get older, the physical abilities decrease, which is particularly frustrating because your brain gets so good! So as you are becoming less technically or physically able, younger dancers are emerging who need the space to perform.
Young dancers are training at a very vulnerable time in their lives, through adolescence, and while they are trying to work out who they are as people, never mind as a dancer. So train the whole person, not just the dancer.
People often argue about this. Obviously one of the skills in performance is acting, and you can't expect every Romeo to really be in love with their Juliet!
Because muscle is heavier than fat, dancers weigh more than you might think, but they are usually very lean.