You do realize, of course, that there is no real evidence to support this idea.
I've heard anecdotes, and of course terrorists will gladly feed this to you if you're gullible enough to hear it. But although "experts" have declared this, nobody has come up with a numerical basis for it, never mind any feasible way to produce one.
What Iraq and Afghanistan have provided is a place for those who are already terrorists to gather in one place. And, apparently, according to al Qaeda leaders, die by the thousands.
Go down a typical list of terror leaders and activists. How many of them were just some laid-back college students until March of 2003?