I question what the risk-weighted expected return for these kids ends up being, and how that compares to more traditional career paths. Thiel himself at least had the fallback option of a traditional career in case his entrepreneurial aspirations didn't work out. It looks like most of these kids already have a top college acceptance in had before accepting Thiel's fellowship, which gives them a backup option in the very likely scenario their "dreams" don't pan out. For kids who didn't have such backup options, I'd really question the rationality of pursing something like this.
I do question the broader movement to push kids from college to startups. I had the option of dropping out during college to work for a startup, and in retrospect I'm tremendously thankful I didn't take the opportunity. There is just so much risk of permanently prejudicing yourself for traditional types of work, and it's just a fact of life that very few startups are going to be successful to the point that it would justify forgoing those other opportunities.
The cynic in me is inclined to believe that the only ones who really benefit from this push towards startups, in the expected return sense, are the VCs. The VCs can hedge their risk between a number of startups to maximize their expected return. An individual startup founder cannot do that. An individual startup founder who is 19, fresh out of college, and has no safety net can do even less to minimize his risk, and has a lot to lose in the likely event that he fails.