If it's a canned kit, replace the yeast, two years is the practical limit. Something more complicated, see above.
I steeped some fresh grains and made a batch from two 8 year-old cans. Turned them into a stout.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk