I have certainly not come across anyone under 21 going to stage 3 and I have closely watched all three of the selections since the scheme started last year. I only know of one 21 year old that has been successful, but there may be others or younger people that I don't know about.
I also know that three 22 year olds were recently accepted onto the scheme after they were deferred at the first stage 3 last year to gain some life experience. In answer to your question then, maybe it could be said that NetJets prefer individuals who have spent some time working in the real world after school & uni.