While YC's structure certainly works better for single people in their 20's, I don't think they started with the goal of appealing solely to that demographic and worked backward. They simply tried to create the ideal structure for tech startups to quickly grow (or die before too many resources have been spent on it). It appears that they have succeeded.
The reality is that neither YC, nor the startup world in general, will work for everyone in every life circumstance. You will always have a more difficult road when your personal life imposes constraints upon you that your competitors (in this case, other founders) don't have. But that doesn't mean that YC was designed to leave those over 25 out in the cold. It means that people who are able to dedicate themselves to their startup for 3 months, with constant access to advice from world-class entrepreneurs and investors, and the opportunity to pitch their ideas to every kingmaker VC in the tech world, will have an astronomically higher success rate than others.