It will be interesting to see what average age represents a skill decline for high level LoL and DotA2 players.
We're seeing a lot of churn still, but at least for now I attribute that considerably to the continuously expanding marketplace for the game. Put more simply: the best players from 2012 were the best players out of, oh, 3,000,000 players, let's say. The best players of 2014, however, will be the best players out of 10,000,000+. As such, the best players from 2012 may be the second or third or tenth best players in the larger player base.
Distinguishing between that effect and skill decline is nearly impossible for now.