Agger and Carra is still a great unit, even if neither has really hit peak form this season (although... I think Carra has had some excellent, and some useless performances, so maybe he has briefly got there). I think with Agger its form, several years with long term injuries, and I think, if he can stay basically fit, he'll get back to his best. I honestly wouldn't be surprised to see Carra keep going for many more seasons to come also. Certainly Rafa doesn't seem to have any problems playing defenders well into their 30s if they are physically up to it.