DT's and interior players usually play well even at advanced age. While it would hurt his pass rush potential, to clog up a lane doesn't require youthful athleticism.
And you may say, well I'd prefer a younger, cheaper body. I'd counter that w/, very few young DT's come in and make a significant impact year 1. It's like the TE position, takes a year at least.
If this is a reload year (which I think it is) adding more veterans with say playoff experience would pay more dividends than drafting young players you're going to bring along slowly anyhow.
We could go after the DT from Redskins who's younger and more of a rush threat but he's going to cost a pretty penny