Even Nobby is only 36.
How many players reach 40 year old and are still playing, at any level? Nevermind somebody like Barton who's has visibly wained physically over the last 3-4 years.
Dean Windass was about 40 and still played a few in the Premiership for Hull, and i'm guessing theres a few kicking about in the lower leagues (probably reserves mind) somewhere.
It's pretty clear that he won't play until his 40 like, but nobody, not even himself, realy knows if he will be physicaly capable of doing so. He's hardly deluded for suggesting so (I know you never said so).