"We'll see. I'd like to be able to play until I'm 40 and take part in another World Cup, but you have to assess the situation year after year," Beckham told Saturday's Sport Week magazine.
"When you reach my age, things start to slow down. I've always said I'll always be available for England. If they didn't call me up, I'd do everything to put myself in the condition to be recalled.
The former Real Madrid and Manchester United player added: "The moment may come when I say 'enough', but as things are now, I don't see it as possible."
Beckham has admitted he is no certainty to make England's 2010 World Cup party.
He rejoins AC Milan on loan in January to try to boost his chances of making Fabio Capello's squad after helping the Los Angeles Galaxy reach the final of this year's Major League Soccer Cup.
"There are very good players in the squad and those who want to be part of it are just as good," said Beckham, who worked his way back into the England set-up with a spell in Serie A last season.
"The competition will be ferocious and that's why we have to give our all every time we play.
"We all know what Capello is like and how he reacts if you are playing well or badly. In the first case, you have a chance; in the second, you don't.
"That's why it's important for me to return to Milan and put myself on show. Capello always said I had to return to Europe after December if I wanted to be at the World Cup."
