How old is too old for the NFL?

The Sporting News just had an article about how 37 is basically the year when most players are too old. Obviously, though, it's an individual thing. If you think about it, most athletes have been playing literally since they could run! So, depending on how well they took care of themselves for, say, 30 years in so in terms of stretching and stuff, that makes a huge difference! Wow, sorry about that run-on, sentence.