Given that the element of selling is a no go tabboo zone, there needs to be SOMETHING beneficial to investing in youths.
At the moment you could train up a brilliant driver, only to not afford his demands when it comes to race.
As suggested, maybe if they have raced three lower tiers with you (ie, 12, 15 and 18) then they should drive as a main driver for at least one year on a reduced salary with consistent high morale no matter the result.
This means he may be shit, but at least the team is boosted by using him from your home pool as a worst case scenario.
That in itself is a reason to use your own youths, even just as a no.2 driver to boost the team dynamics.