I would love to know the answer to this question.
I am certain that the average age is increasing at a rapid rate. Consider some factors:
- Conversions are way, way down.
- Half of the young people leave.
- Defections are up
- Every year, everyone gets a year older
Since most people change their religions when they are fairly young, these trends mean that they are losing a lot of young people, and not refilling them with new converts.
I would bet that their average age is rising by about 1.5 years annually.
In my area, there are very few youths. Many rural congregations are filled with 70+ members. Every year another rural congregation is disbanded.