Argonath is aging community. As right now it seems that medium age seems to be 17 then I am very aware, that when in 2006 Argonath was opened, the median age was around 15.
Also we can see, that Argonath has reached it's optimum and maximum median of age. Because from the poll you can already see, that the player amount is rising until the age of 17 and then starts to decrease (with the current anomaly of age 18, what I believe will be corrected soon).
Also you would have to make a difference in the age 20+, because it contains age 20 and above, if you would take them also separately, you would see that in around age 22-23? there is our first gap, where there would be no players.
The decrease of players of 18 and beyond is caused by the changes in life. In a lot of countries people start college or Universities at the age of 18 what leaves less time to playing and therefore decrease in players. And after the college comes work and change in priorities etc.
Why is so few players with age ...-13. Well mostly because until then boys like outside activities. Play cops and robbers outside, run and run and run.