Historically this has been people in their teens and 20s and older people whose kids have grown up. But I think this is changing. Yes in your 40s and 50s it's difficult to have the absolute (almost obsessive) focus required to develop software (Tim Bray and Dave Winer are the exceptions that prove the rule methinks). However older people make great spec writers and product managers and can contribute significantly in these and many other areas. Speaking personally I still get the urge to code even though I was never a programming guru like Winer and Bray and occasionally indulge but these days I know I can contribute much more doing non code tasks.
Is the only good stuff coming from young(er) people? Of course not -- Dave Winer will continue to stir the pot with new technologies and approaches.