I've interviewed at a few YC companies, and the issue with really young companies is that they don't know anything about running a company. Experienced people don't want to work for a 27 year old, and so it's a bunch of people who don't know what they're doing, making promises out of ignorance and not delivering.

I'm tempted to think that YC knows better than I do here, but my experience suggests this is a bad idea. The worst managers I've had were inexperienced, and someone who's 22 won't know when they need someone experienced, or how to talk to customers. Many of them may be too nervous to act, and since they can apply under the safety of also applying for jobs they may not be as motivated. I suspect this is just a ploy to get more Stanford grads to apply but I'm not sure it's the best way to go about it.