It depends on the pain he's going through. In my head he can play until 40, because he's that good.
But as GreenFaith says, we need capable bigs to take some pounding off KG. If he has to deal with all those 7-footers without major help, he'll be out of the league too soon imo.
Not sure who, but it better happen sooner than later.
It makes sense to both KG and Pierce to play this year out and see what's our team next year. This year's playoff run will be very useful to guys like Green, Bradley and Lee, and if you add healthy Rondo and Sully, that could motivate the veterans to stay one more year.