1. Median Age
Experts will tell you that if you want to know if a population is in growth or decline, the best single number to look at is the median age of the population. I have extracted Aust census data and calculated the following approx median ages:
Year 2006: Aust population: 37.0 JW's: 38.7
Year 2011: Aust population: 37.6 JW's: 41.2
Year 2016: Aust population: 38.1 JW's: 43.9
For complicated reasons, I am not confident of the last decimal point, in the above. However, I am confident that the numbers broadly represent the situation.
To put it in words, over 10 years, that median age of the Aust population rose slightly (1.1 years), but the JW median age went up substantially (5.2 years). A median of anything over around 40 would normally indicate decline, and the higher the number, the faster the decline.
2. Age Cohort Detail
Here is a breakdown of JW's from the last 3 censuses, into 5 year cohorts:
Comparing 2011 to 2016, these number show that in every 5 year age category below 60, JW's have less adherents. Aust has a high immigration rate and the numbers look worse for the Borg if I extract out those from 2016 who were not in Aust in 2011. (It is not hard to do, but I suspect I will just confuse people with too many numbers if I set that out.)
3. The "return to Jehovah"'s have stopped returning
Another way to look at the data is to see what has happened to each cohort over 10 years. This involves reading the above chart diagonally, instead of straight down. Again, I haven't included the data for the sake of simplicity, but the increase in most cohorts between 2006 and 2011 is around the same number of JW's who in 2011 reported they were living outside of Aust in 2006 (ie, most of the increases were immigrants). The major exception is the significant increase in the 1 to 4 cohort.
To explain, the above chart shows that in 2006, the number in the 1 to 4 cohort was 4058. in 2011, these kids were in the 5 to 9 bracket. Note there were now 4857 of them. That is 799 young kids drawn in by their parents. (Actually there were only 404 of them, because 295 of those kids were immigrants.)
Maybe the Borg is having a lot of success recruiting young families. I think the more likely explanation is that a lot of the teens and young adults who left, started drifting back into the Borg when they settled down and started having kids themselves. This "return to Jehovah" group was probably the difference between expansion and decline in the past.
This changed in 2016. Doing the same exercise comparing 2011 to 2016, the effect is still there, but it is very small. For the 0 to 4 bracket, there is only an increase of 217 kids, which is explainable by immigration.
This is the reason I say in the title "the 30+ group aren't returning to Jehovah any more".
4. It is not just the young ones who are leaving
If you do the same exercise of following a cohort from 2011 to 2016 (ie reading the numbers diagonally, instead of straight down) you will note that they are actually losing numbers in every single age cohort. To put it another way, the reason the numbers for the 60+ groups are holding up is not because they are staying, it is because the younger ones are getting older and taking their places.