Its funny you say "for grownups" while more than half the people in YC were 27 years old or older - certainly adults out of school and doing all the things grownups do: marriage, kids, taking on debt to buy homes, etc.
I realize that. It was more a positioning statement to separate it from YC. There are lots of other YC like programs too. The idea being to identify talented people who may not feel comfortable managing their constraints within existing accelerators. More importantly, to your point, I don't think that's an attractive target group for investors.