Note that I said implicitly. There is no explicit age discrimination that I am aware of, but the entire program is structured in a way that makes it horribly impractical for anyone who isn't a bachelor/bachelorette and childless. You must move to the Bay Area, put in family-unfriendly hours, and live on a quantity of money that would make it impossible to live in the Bay Area with a family (assuming you don't have a lot of savings, a trust, or a high-earning spouse). It just won't work for a huge majority of older people or people with families.
The implicit assumption is that all the best talent and ideas can be found among those who pass this implicit filter function. Anyone over 25-30 or who has children is "washed up."
... which creates room for an accelerator to test that assumption. What if -- gasp -- people who have been doing this stuff for a long time are actually better at it?
But as you say, nobody is even really trying to compete with YC within their own segment let alone exploring alternative modalities.
Edit: another one would be to question the assumption that sole founders are intrinsically doomed. They are certainly at a disadvantage, but that'd create an interesting niche for an accelerator that helped them to compensate for their disadvantages if they otherwise had really great skills and ideas.