Midgets with Jetpacks (great name for a blog, BTW) does some nice statistical analysis for you on running backs in their thirties.
As you can see, the trend is promising for older running backs. Since 1999, each season has had at least 2 running backs with 1,000 yards or more (with the exception of 2002). And, since 1998, with the exception of 2002, there has been at least one running back aged 30 or older who has topped 1,200 yards each season.
There is much more to to it than that, go check it out for yourself.
Just another reason I expect Shaun to have another great year.