Ask anyone who graduated from Harvard (undergrad), HBS, HLS, etc. about the Harvard Extension School and you'll get the eye-roll I'm talking about. I think in cases like this it's less because of the adult part but more because of the payola part -- anyone can sign up for HES classes provided they pay.
What I think is potentially interesting, and frankly am surprised isn't offered yet (or at least not widely), is the idea of continuing one's education into adulthood through the same university.
My personal motivation: When I was 19 and a sophomore picking a major, I chose what interested me at the time - business & psychology. I loved what I learned and am grateful for my college education.
But here I am 18 years later and my brain and desires have changed. I'm rabidly excited about programming and CS topics, and have done a pretty decent job of teaching myself through various online resources over the past few years.
My alma mater should capitalize on this -- they should offer a "supplemental degree" program where I can give them money, take classes (remotely or on campus), do the work, take the tests, and get a CS major supplement to my previous undergrad degree. It would maintain the full authority of the university and be a full bachelor's degree, just completed almost 2 decades after the first bit :)
I can't imagine I'm alone in wanting this sort of thing...