I do think experience plays a big factor (not enough for an average player to beat an awesome player, but substantial nonetheless

)
There are a few issues with that approach however:
First, I'm not sure how much experience playing in the U21s gives - playing them from 18yos on may give 1 or 2 levels of experience, but is 1 season of slightly better performance enough to justify sacrificing 3 seasons for?
Second, there is no guarantee that the best 18yos will be the best 21yos - alot of managers may get training wrong/ go inactive etc.
Third, I think Wezagaza raises a good point, and its one of the main reasons I didnt turf out all the 21yos when our world cup campaign ended. Alot of people look forward to getting a player in the U21s, and those that have trained up the best guys (like Wezagaza with Cranshaw) deserve to be rewarded for that.
And finally, it must be remembered that the U21s are also a feeder team for the seniors, by playing guys like Cranshaw now, they may be that little bit better when they reach NT level.
Of course, there is also the issue of whether an U21 coach, after two seasons of poor performance with 18 and 19yos, would actually get voted back in to reap the rewards

.