Aria-,
This 19 year old was in Australia, where I am originally from. As stated above, I have not flown with any one less than 27 years old. (NJE)
Previously to be considered in Netjets Europe, you needed a minimum of 1,500 hours. Since you can't get a CPL until you're 18, this means that no one worked as a pilot for Netjets Europe at age 19.
Now the Be 1900 FO requires 700 hours or so minimum.
So, at this point in time, no, no 19-20 year olds have been successful at NJE. The cadet scheme is a new thing started last year. NJE has been operating since 1996. The US parent company, even longer.
You probably would not need "a couple of years" to step away from the school mentality, 6 months working full time would probably be sufficient. The money will come in handy too. You'll need it.
As stated previously, nothing's stopping you from applying. We're just trying to offer some advice about the reality. Remember, if you fail stage 2, forget the NJE cadetship. If you still want to work for them, you'll have to get 1,500 hours min. If you haven't graduated from school, I would concentrate 110% my efforts on getting the best grades possible.