With Urlacher already out and Peppers, Briggs and Tillman on the wrong side of 30, how is it wrong?
They are talking about the core, the heart of the defense.
It doesn't mean you can't get younger in some spots, but as of right now, your core, your defensive leaders are Tillman, Briggs and Peppers, and who knows how long they will be able to continue to sustain? Of the youth, there just isn't enough there yet to say "ok, here is a young core that is ready to take over".
So the people talking about the defense aging have been and continue to be right.