Generally only the really professional church choirs have members in their 20s and 30s. If you want to sing with younger people you generally have to seek out people you know who sang in university, and found a way to keep it up.

I'm the youngest in the choir I currently sing in (a men's choir with mostly priests and farmers), and I was in the other choir I used to sing in too (a mixed choir with mostly doctors and nurses). Somebody has to be.

There's a lot of youth and children's choirs too, so it's not as if it's dying out. It's just that people in the middle tend to have less time (the doctor choir was fairly middle aged, but you know how doctors are...)