And after you get your helmet adjusted perfectly, sew the straps together. Some older helmets like the Bell Biker were reasonably good not quite up to current standards. Others were poor performers. If you still have an old helmet without a styrofoam liner like the Skid Lid, we recommend that you replace it immediately. Most manufacturers now recommend that helmets be replaced after five years, but some of that is just marketing.
Deterioration depends on usage, care, and abuse.
If you ride thousands of miles every year, five years may be right. You must replace the helmet after any crash where your head hit. The foam part of a helmet is made for one time use, and after crushing once it is no longer as protective as it was, even if it still looks intact. Plastic shells hide the foam damage. If you can see or measure any foam crush at all, replace the helmet.