Over the last decade we've seen major tech growth in two areas: mobile computing (computers as fashion accessories) and social networks. Both happen to be areas where youth culture has historically served a role as early adopters. If you're looking for historical perspective to figure out when 20 somethings were given the keys to the tech kingdom, it happened over the last decade when tech companies started using youth culture to drive consumption and "innovation".
Why do we need companies full of 20 year olds?
1) Because 20 year olds develop products for other 20 year olds. Most of them don't have the experience, wisdom or maturity to do anything else and they are fundamentally still trying to please each other and define themselves as a generation.
2) 20 year olds have a ton of energy, health and are willing to work ridiculous hours on bad code for lower pay.
3) Making millionaires out of a few 20 somethings every year helps feed the fire.