The oldest I've used to date: Ilford Soft Gradation Panchromatic glass plates of a pre-WWII vintage. Badly stored in a loft, and blotchy almost unusable results. Also have a stash of Fuji Acros 100 that is three to six years out of date - This stuff has been frozen since purchase and is returning excellent (in my opinion) results.

Relatively slow colour films *should* still be OK as long as they have been stored appropriately, but I wouldn't consider paying a premium price for them.