So, you are arguing that 4e is already in senescence?

The difference in age is minor, so if 4e has already peaked then it is
really in trouble.
For what it is worth, I blame 4e's releases more than I do any 'maturity' - what they were releasing was not what people were buying, and they were saturating their market.
Poor choices for release are much better for 4e than it being 'mature' - it is something that the folks at WotC can correct. If sales have peaked because 4e is 'mature' then there is nothing that can be done to save the current property, and it is a very young property compared to how long 3.X lasted.
The Auld Grump
*EDIT* Just in case any one is likely to misread my post - I do not think 4e is 'mature', 'aging', or 'senescent'. I think that it is in a slump, and may yet go back to being king of the hill, even if it is not the game for me.