They're not ideal. A few brown spots here and there on a bunch of green beans mean they're getting a little aged, and won't be the freshest beans you'll eat. But it doesn't mean you can't—or shouldn't eat them. Like any other food products that aren't spoiled but are past the peak of freshness, it's best to use them in dishes with bold flavorings. Not that they'll taste off otherwise, it's just that you're not going to have a pristine bean experience.
Look for visual cues to tell when your beans are getting old: brown spots, withered tips, and bean seed shapes that protrude from the shell mean the end is near. If they've become wet or slimy, they're shot. Keep beans fresh longer (for up to a week) by storing them in a sealed zip-top bag in the crisper drawer of the refrigerator.