I've worked both in orgs with "greybeards" and in the org I've founded along with many clients. There's plenty to be said for veteran levels of experience, but this is something that's only really needed in a leader, as this experience can be communicated and demonstrated to others. A fool learns by his own mistakes, a wise man by those of others. I suppose this makes me a fool, but a fool who's decided to help others be wise, and to harness those several decades of experience in such a fashion that it benefits all involved.
So yes, you have a point in that years of experience are indeed valuable, but I would argue that this is not a prerequisite to be an exceptional engineer, and that in fact truly exceptional engineers arise through memetic experience, and a thirst for knowledge.
It's true that I speak from personal experience, but then again, so does every scientist who finds a repeatable experiment. I'd be happy to be disproven, but until then, I'm equally happy to treat what I find to be a valid theory as fact.
Young, passionate individuals make the world go round. You only have to look to the great leaps and bounds of science to witness this in visceral actuality.