I don't have the data to back this up, but my general impression is that injury rate graphed out by year would look like an inverted bell curve, with injuries being more likely in the first couple of years, tailing off, and then becoming more common again as the players approach their mid 30's.
This should actually be expected. By all accounts, the difference between the physical toll of playing in the NFL vs. college is an order of magnitude. Makes sense when you consider that you're now exclusively colliding with the biggest, strongest and fastest 2% of recent college players.
Many players discover that while their bodies held up under the strain of NCAA Division 1 football, they start to break down under the demands of the NFL. I remember one orthopedist quoted, I believe in the Globe, referring to it as the "tissue issue." There's a great deal of variance from person to person in the strength and elasticity of our soft tissues -- our ligaments, tendons, fascia, nerves, etc. that results primarily from one's genes. Environmental factors, like poor developmental nutrition, can have a detrimental effect, but there's really nothing one could do to improve it, short of getting oneself bitten by a radioactive spider.
A lot of players end up washing out of the NFL not because they were unskilled, but because some part of their bodies just didn't have enough tensile strength. I believe, on this board, the common diagnose is being "made of glass." These guys generally wash out of the league in a few years, so as you start looking at 3rd, 4th, and 5th year veterans, you're looking at a population where the innately injury prone have been weeded out, and thus, you'd see a decrease in injury rate... until they get older, and between the amount of cumulative wear on their bodies, and a naturally diminishing healing rate, and the injury rate rises again.