I don't work in Silicon Valley, but if I did...

I'd find the idea of hard-partying 20-30 something year olds indeed a ridiculous stereotype.  I'd find it a ridiculous stereotype anyway a) because there's lots of people my age over there... that is they're "grownups," and b) every engineer in the course of their work knows or should try to know how they're going to change the world...unless you're an engineer in the defense industry (where it's kinda sorta obvious how you're going to change the world).