Well, I have to disagree with you, redtooth. If the potential youngsters are exclusively trained from 18yo, they can easily dislodge the oldies after they turn 22. With due respect to the current NT players' coaches, they are not exactly world class, yet they are now too old to improve and also hit the potential cap already.
Take a look at some other countries' NT and you'll know what I mean. Those 22 yo monsters earning >100K salary went straight into their respective NT. The difference is mainly due to these countries have a group of devoted clubs that go all out to train their ATGs and HOFs from Day 1.