Say you have an exceptional trainer or above. For a U21 ID, IS and RB at 13 min etc.. how long does it take to then move that player into the NT. That is if the player is good enough as DR and JS would need to be worked on. If you had a PF how much OD would you train them up to have for the NT?
It would take roughly 2-3 seasons just for the inside skills, and that's ignoring SB. Economically its probably better to have two skills higher than the others, otherwise the player becomes very expensive. Some would argue you should leave one skills much lower but I'm not especially in favour of that.
Generally its rare to see young players on NTs. Our youngest is Big Mac MacAusland, who has actually been on the NT for several seasons. He's a freak however.
A much usual progression is that of Robert Osman who plays for Tatatallfellas in the EBBL. He could be on the NT now at 25 (he isn't because he played for a bot earlier in the season so had poor game shape) and indeed he was last season but he still needed some training to be the equal of our other big men. From that I think we can generalise that it takes about 3-5 seasons to train a player from u21 to NT. Of course there will always be exceptions. Some players have very high starting skill counts.
For OD, I see I've put 7 as the requirement on the NT page. I'm not entirely happy saying that, I regard it as the minimum really for a PF, even at league level. I'd much prefer at least 10, but the taller the player the more difficult it is to train OD. (Normally the taller the player the more suited he is to C rather than PF for this reason.)