Older bread more sour?

Hey folks,

I have only around a month or so of experience with sourdough baking, so forgive me if this is a foolish question, but I couldn't find it elsewhere on the boards.  I have noticed that my sourdough loaves seem to get more sour as they age.  If I eat some the day of baking or the day after, the sour taste tends to be a bit milder than I want.  Around 2-4 days seems ideal.  Then, around 5 days, it starts to get a little too sour for my tastes.

Am I crazy, or does this really happen?  I thought at first that I was perhaps just getting sick of my loaves before I finished them, but I've repeated the process several times.  My sourdough breads have been mainly moderately-dense rye breads, and while I understand that the flavor of dense ryes typically matures over a day or so, I haven't noticed any development beyond that period in non-sourdough loaves.  Perhaps it's just apparent in sourdough?

-shakleford