Does water-based paint expire?
I don't have a specific answer for you but all of the data sheets from Behr I've come across seem to suggest that they have a shelf life of two years (IF unopened).  For example: http://www.behrpro.com/cma/BehrPro/Marketing/Products/TDS/7050_27050.pdf 

I'd say 6 years is pushing your luck and I wouldn't recommend using it for highly visible or large projects... or project where you need to blend the color with something else.  Maybe you can use it for some minor trivial touch-up projects?