The Cardinals were lambasted for a few reasons for the Holliday signing, whether if it was for the amount of money, the years or that no one was bidding against them.
A poster in the Fangraphs article about the Holliday signing linked a very good article from Hardball Times.
If you scroll down to the middle or so, it states that players who played 10 years and had 5,000 plate appearances 1950-2008 have a very good decline, and 1980-2008 often had peak years between ages 30-33.
From the article:
In the pre-1980 era, for players who have at least 10 years and 5,000 PA in MLB, the aging curve is pretty symmetrical around a plateau stretching from around 27 or 28 to 32. From 21 to 27 or 28 is almost a mirror image of 32 to 38. In the modern era, the player with a long and prosperous career peaks at 30 stays relatively stable until age 33, declines gradually (around two or three runs per year) after that until age 38, and then declines by around five runs per year after that.
Holliday is on pace for that and has six years so far with almost 3,700 plate appearances.