Only the ones who are physically up to it.
Jersey Flegg was under 19s back when I wasn't good enough to make it. SG Ball was under 17s and Harold Matts was under 15s.
Flegg has never been higher than under 20s. But there used to be an under 21s (called Presidents Cup) below reserve grade, right up until sometime in the 90s, and until the 80s there was also an under 23s.
I don't think there's enough money for three grades of paid players (or rather, the most influential players don't want to share that money).
Any players under 20 who aren't physically up to it shouldn't be playing.
Of course there is enough money. There was before and the NRL should have more now with the TV deal. In fact there isn't really a need for U23s. Keep it as U20s but have reserve grade as the main second competition.