My observation as well (midwest US). I think it is caused by a combination of things.
2/3 of children raised in the religion leave.
very few 20-40 age couples join as new converts (unlike the 70's and 80's when you would see whole families join in)
They do very little/close to nothing specifically for the young people to have fun (many churches have youth ministers, special activities for youths, etc)
The relatively few young people who stick around and remain active (uber pioneers, etc) may get married but usually do not have children.
I think there is an underlying demographic trend going on also, at least in the US, where people are waiting to a later age to start having children, and having fewer children.