Obviously, age is already affecting them as they're on the middle-age cycle now -- one album and major tour every 5 years; whereas, in the early 80s it was more like one album and tour every 16 months.
I suppose they'll just continue like this, slowing down, until old age finally stops them. Like others said, I just cannot imagine the 4 of them being healthy and able and just quitting music -- seems out of character. Having said that, I'm not sure I want to see them old and wrinkly and playing "I will Follow" as Bono limps across the stage and Adam hits the strings from his wheelchair. I would guess they'll keep up the stadium-thing for another five years or so, and then they might start to scale down the operation (either by choice or by necessity, if they lose popularity as may already be happening a bit). But in any case, i don't seem them quitting music altogether, ever.