First, here is the 18 & over division.
The trend here is similar to the overall trend, the drop here being just over 3% since 2013.
Here is the 40 & over division.
An explanation for this might be that as some of the 38 and 39 year olds have turned 40, they've elected to only play 40 & over and there have not been enough new under 40 year-olds joining to fill the void. And those that turn 55 seem to continue to play 40 & over. Also, the 40 & over division was new for 2013 so perhaps it took some a year or two to get on board with it.
Let us see if the trend continues in 55 & over.
Sure enough, the trend continues. This group is up over 4% since 2013. As the 53 and 54 year olds have turned 55, they keep playing at a higher rate than they get too old or injured and stop it would appear.
Given all this, it would be interesting to see what the average age of these players is. Alas, I don't have that data, but I would hypothesize that the average age is increasing which would indicate that existing players keep playing as long as they can, but the new younger players are not joining and playing USTA League enough to replace the older players that do end up not playing at some point.
What do you think?