211500 - I'm talking about juniors through to U/20's. I'd can U/14's Nationals for just the reasons you're talking about - lots of post-pubescent kids who are highly-skilled and coordinated but have maybe another inch growth in them. Rarely are they already tall enough. I feel for a lot of the early-maturing boys who are stuck in the post the minute they have their first shave, only to find out they have topped out at 5'10" with post skills only.
To the how long do you persevere you have to ask how tall? Mark Eaton played in the NBA simply because he was 7'6 (or whatever) and that changes heaps. If he was a mere 7-footer they wouldn't have persevered for so long!
If you want to run a stereotype to the end, we need to look at teaching talls in a different way. These are kids who stand out all the time (and often shy away from that attention - ectomorph (tall, skinny, often intelligent) body types are also often shy), often because they lack the coordination of their peers. They are expected to succeed at basketball simply because they are tall, so we throw them in with everybody else and try and teach them to dribble in a one-on-one drill with a guard who makes them look stupid. They miss shots from so close to the basket we can't believe it but they never take an unguarded shot (and generally never one without contact of some sort). They are continually targeted by the opposition coach - whose general solution is to physically intimidate them and we wonder why they don't develop quickly. There's nothing we do to help them develop confidence or skill (not to mention getting on forum's and bagging their selection over other, more skilled players!).
As I said in an earlier post, I have a tall daughter. I watched a game last year where the opposition coach wouldn't let his big (6'7") go against my daughter (6'3") who was more athletic, and so had a dispensable player 'tag' her all over the court. Not a single foul was called even though it was effectively a Hack-a-Shaq treatment (without the foul shots). Had I man-handled the referee that way in the street (I was sorely tempted I must say), he'd have been most offended, yet we expect developing bigs to handle that because they are bigger, often less strong, often less confident, but taller.