Critical mass of the audience, most likely. Early to mid 20s overlaps when people are most willing and able to form their musical and artistic identities as consumers of content.

It's pretty rare to see people older than their mid 20s hanging out in new/hot venues with the latest artists trying to build a following and living/breathing their craft. There's only so many years you can pull it off both as a member of the audience and a creator.

A few years ago I was a minor artist (more a hanger-on than anything) in the EDM scene of a major city for that back when it was the hot thing, and was around a few folks that made it. It couldn't have happened for people in their late 20s or older. That particular grind requires of fueling 18-22 year-olds appetite for drugs, alcohol, and rhythm from Thursday through Sunday, every week of the year. When you factor in the reality of rent and other costs of living, dueling roles as promoter, party goer, content creator, and performer, it really is not sustainable unless you're the azimuth of your craft and an age where you can balance those costs against your future.