"Development" never stops. A swing requires constant attention. A good player coming into a D1/D2 environment is often outmatched at the plate. I've watched kids ineffective (< .200 ave) as freshman become solid hitters (>.300) as juniors. Lots of work in the weight room and in the cage. Defensive fundamentals work never stops...you'll see it in pre-game warmups from some of the best college teams in the country. As younger players become more consistent with fielding and moving the ball around, it can be blended with the finer points of making plays and executing more complex defensive schemes.
I'd guess that most coaches who've gotten to 16U have taken on an athletic-looking "project" and had it not work out. Baseball/softball are weird games. Being "athletic" is a nice foundation, but I've seen many players at every level who passed the "eye test" and never seemed to fully get it. Conversely, there's the kid who doesn't look impressive at all who soaks up everything they're taught. The willingness and ability to work outside of practice is often the difference between success and failure, but sometimes it's just not their thing.
I'd guess that most coaches who've gotten to 16U have taken on an athletic-looking "project" and had it not work out. Baseball/softball are weird games. Being "athletic" is a nice foundation, but I've seen many players at every level who passed the "eye test" and never seemed to fully get it. Conversely, there's the kid who doesn't look impressive at all who soaks up everything they're taught. The willingness and ability to work outside of practice is often the difference between success and failure, but sometimes it's just not their thing.