I wouldn't say young bahais are forgoing marriage entirely. In fact, I felt like when u was a bahai, the other young bahais were obsessed with marriage and finding their perfect mates. A lot of them got married young (early 20s), and were divorced by their late 20s/early 30s.

I also found that because living together before marriage is forbidden, a lot of young bahais (that came from bahai families) married young in order to live with their partners.

One possibility why the bahai community is so "old" is that bahais are growing up and not staying in the bahai faith. If they are getting married, they are likely marrying outside of their religion.