For those wondering about how the previous generations were able to do it, the fact of the matter is that they weren't. Craig McDermott missed 3 years of Test cricket early in his career, Merv similar, Jason Gillespie famously played half of the Tests available in his early 20's, Shaun Tait (say no more), Brett Lee had stress fractures before and after playing his first Test, etc. Just about every quick Australia has produced since I've been following cricket has missed large amounts of Test cricket prior to their 25th birthday.
The reason why the newer guys seem to be missing more than usual is because of the sports science being rubbished here; they're having minor injuries being managed by missing the occasional Test instead of bowling their way towards injuries which will result in a couple of years of recovery. Even Glenn McGrath, with his biomechnically excellent action and having played Tests relatively late (I think he was almost 24), missed an entire season of cricket twice in his career and that was most definitely managed rather than waiting for him to snap something entirely. It's just smart management of assets.
What's not smart is waiting for season-ending injuries to occur and digging deeper into the well of available quicks, especially in the era of back-to-back Tests/series. What's happening here is that by giving all of these guys a couple of Tests here and there, in a couple of years time when their bodies settle down, we'll have 8 or more quicks with 20+ Tests under their belts still bowling as quick as they did at 20 with much longer careers ahead of them and we'll all be looking back and wondering why it didn't happen sooner.