the average post 2000 is less than 10 years.
Well it would have to be, wouldn't it.
It's only 2015.
(2015 - 1999) / 2 = 8.
What did you expect?
Also, you're only asking people that have left to participate so you are automatically excluding everyone still in. Although, discounting for the slight decreasing trend in baptisms over the past decade or so, we would expect the average length of time people have been "in" the religion since 2000 to be about the mathematical mean also, but for the opposite reason.
It would be more meaningful to know what percentage of active JWs have left at any given time. The way the WTBTS reports data makes it extremely difficult to know this.
There was a very detailed discussion about this general idea about 4 years ago back before SLIMBOYFAT left (welcome back) and everyone here was still on speaking terms with Cedars.
As I recall, this was one of the first threads where Cedars got his knickers in a knot because people were calling him on his flawed statistical analysis:
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The discussion really gets interesting around page 7.