Considering how much pressure NRL coaches are under, and the amount of testing / training players receive, I seriously doubt any player will debut at 18 or 19 that isn't considered to be ready!!
And this is probably supported by history, just a rough guess (without looking at the history books), but it seems most teenagers that have debuted over the last few of years seemed to have been ready and have remained in the top grade, Cleary, walker, Johnston, Moeroa, Mitchell, Mitford, cotric, Pritchard, critchton, Scott, .... just to name a few, all seem to be headed towards a long term NRL career.