Federal law regarding age discrimination doesn't really apply to professional football players, mainly because the Age Discrimination in Employment Act does not protect workers below the age of 40, but also because it would be a hell of a thing to attempt to prove in court.
But if you scan the list of starting-caliber players who are still lingering on the free-agent market one month into the new league year, you'll notice that almost all of the proven players who remain unsigned have one thing in common.
They're old, at least by NFL standards.