Among amateur singers, what vocal ranges are most common?

I’m working on a project which is to be performed by amateurs and I have two pieces of information I’d like to get a handle on.

First, soprano, alto, tenor, baritone – do they occur with about equal frequency among adults? It seems true basses are rarer. Am I correct that ranges lower as people age?

Second, for a child, is range expressed the same way? Would you say that a young girl was a soprano?