The trend here is not quite as clear in terms of finding the peak, but note the cliff at age 30. Let's try slicing it this way. This is a much different picture than what we saw for running backs. Young receivers are more likely to improve than young RBs. My guess is that this occurs because young RBs are more likely to already be good than young receivers are.
Old receivers are more likely to decline than old RBs.
I can't think of a good explanation for this, but the data is compelling. Something to think about for you dynasty league types. Quarterbacks I expected to find no clear trends at all for QBs. It's clearer than I anticipated. This looks fairly similar to the receivers, except that the peak appears to be a little longer and a little later.