I think we are all afraid of Farrior instantly deteriorating much like Porter and Gildon.
Howver, the major difference is that an OLB's disappearing act is much more noticeable than the SAM linebacker spot that Farrior plays. He's not required to rush the passer most downs, so his run stopping abilities can be utilized well into his 30's.
Look at Zach Thomas, that guy is as old as Jesus and is still a tackling machine.
I'm really happy we have Farrior locked up until he retires.