With the sum of human knowledge expanding rapidly, it takes more time to get up to speed -- hence more years of education are needed before one is in a position to make a contribution. But the same rapid expansion of knowledge probably means that the intellectual capital people acquire in those years of education has a shorter half-life. People may be living longer, but the lifespan of useful knowledge is getting shorter. By the time you're in your 50's, at least in technological fields, your intellectual capital has passed its "use by" date. And by then, it's too late to retool -- the opportunity costs of going back to school in your fifties are huge.