He could play well into his forties if he keeps it up. There are rumours that Inter will offer him a contract extension after his superb form of late.
Who's the world's oldest active footballer so far? I know that Dino Zoff was 41 when he won the World Cup with Italy. Roger Milla was even older in the WC -94, but he hadn't played in a top league for over a decade at the time so he doesn't really count.