Check out
THIS article on the degradation of plastic from UV radiation. Ultimately, it depends on the quality of the plastic (specific material type and types of stabilizers used) and how the plastic is stored. I haven't personally seen a helmet manufacturer recommendation on a replacement frequency but I wouldn't be surprised. Child car seats have a printed expiration date on them but they sit in hot cars for hours daily unlike a bike helmet that may be exposed for direct sunlight for 2-3 hours at a time. I am always suspicious of bicycle industry tactics to induce spending. Somehow they do a fantastic job..... haha