Up until you are 21, your body is still developing. The problems of building up muscle in areas before your body is structurally ready to handle the extra muscle is don't happen until later in life, but not much later in some instances. There's also the added problem of different rates of development in various body areas. For example, the musculo-skeletal system on one side of your back maybe develop earlier than the other. You'll train and your muscles will adjust for this. Then you grow and your muscles affect this growth, sometimes causing bones and ligments to grow incorrectly and/or in the wrong shape. his is but one example of what can and DOES happen to your athletes and cricketers, particularly fast bowlers, are the same. Bowling fast for too much at a young age can cause stress fractures by virtue of the fact that your physiological age doesn't match your real age, regardless of technique too.
The facts remain that no-one under 21 has fully developed. Either some parts have gone ahead of others or some have lagged behind. This DOES cause problems if the players train and play too much. It really is as simple as that.
In my opinion, no-one under the age of 21 should be able to touch anything heavier than toning weights and these directives are spot-on the mark as well I think. It's not an issue of fitness or burnout at all. It's an issue of musculo-skeletal development and if restricting these kids from bowling means that they can play at their peak longer and also help them when they get older, I'm all for it. I know far too many mates of mine who were potentially great sportsmen but because they were onto the weights and heavy training before they fully developed, they're not in real trouble phsiologically speaking. Stress fractures in backs and feet, regular ligament damage, knee recontructions etc. they're all part-and-parcel of boys who had too much, too young.
Sure you can only bowl 6-over spells. So what? If you make it higher, when you pass the age limit, there will be plenty of times to bowl long, hot 10+ over spells to come!