A check can still be cashed after six months, but the bank does not have to honor it. Once a check reaches that age, it generally becomes a stale check, which changes how the paying bank may handle it.
That creates an awkward little banking problem. The check may still look perfectly normal, the money may still sit in the writer’s account, and nobody may have canceled it. Yet the person holding it could walk into a bank and hear, “We can’t take this one.”