At least in the UK, the U3A isn't a university per se - it's an affiliation of voluntary organisations, run locally and funded by member subscriptions. Unlike a bunch of teenagers, retired people have a lifetime of knowledge and experience behind them; a large proportion of retirees are perfectly capable of teaching a class or leading a discussion group.
It's also worth pointing out that the US has many excellent community colleges, offering academic and vocational education at reasonable tuition fees. If you're a retiree and don't care about the signalling value of a credential, a community college is an excellent place to pursue your education.