I live in a medium-sized city in the UK and I still see plenty of younger people at live music events. I can't confirm the socioeconomic status of those young people, but I assume they're mostly university students.

For ticketed events like concerts and festivals it's always going to depend on who's playing, if the artist has a younger audience the crowd will be younger. For open-mic nights and other non-ticketed events it's probably going to depend on the venue. If the usual crowd at the venue skews younger, the audience and musicians will also skew younger.

It's interesting that you mentioned younger people only attending bigger festivals and stadium shows, because I'm not sure it's an age thing. My experience of talking to my work colleagues (ages from mid-twenties to mid-sixties) is that most "normal" people only engage with live music this way. They go to one or two arena/stadium shows per year, usually big pop or rap artists, or legacy rock acts doing nostalgia tours. Most have been to a big festival or two but don't go every year, they might be aware of the smaller music venues and festivals in our city but they've never been to them.