Lynch looked slow as hell last year and I can't imagine the Pats using him in any type of coverage situation. He is a complete liability there at this stage of his career (ask Denver fans), however, I'm sure Belichick can use his veteran leadership and his ability to come up into the box on run downs and hit someone 5-10 times a game so at low a low cost I guess its a decent move...
As far as the Pats D being "old", that is more a media thing than anything, a few of the LBs were old last year and Harrison was old. The whole DL is under 30, Adalius is like 31 which is hardly old and Vrabel is like 32 so he is starting to get there, the rest of the secondary is all young dudes. The problem with the D last year was Colvin's injury forced Seau and Bruschi to both start inside, when they had infact been a good platoon feeding off one another for the first 8 weeks or so. Think Pats D is still a solid top 5 D that will specialize in giving up yards but not points......
As far as what the Pats did in the draft and FA in terms of stocking up on D, I liked what they did a lot getting Mayo, Wheatley, Crable, Wilhite. Given Belichick's history of drafting, gotta think 1-2 of these guys become real good players in their system. Also, I agree with Midman that Meriweather will be a player, he just needs to learn to hold onto the damn football (Asante couldn't catch ANYTHING his 3rd year in the NFL, then his 4th just blossomed into a ballhawk)