I caught mumps at around 30, during one outbreak thanks to the proliferation of antivax garbage. Only about half of kids in some London boroughs were fully vaccinated, centered around a couple of the wealthiest areas--and that was nearly 20 years ago. (Meanwhile, I was indeed fully vaccinated as a kid. It's not foolproof, and especially not forever.)

Thankfully recovered just fine without complications, but that really was NOT much fun. Adults are pretty much guaranteed to get sicker, even if they're otherwise fairly young and healthy--and lucky enough to avoid any of the serious complications which are likelier to occur.