Every tire is stamped with a manufacture date, and tires are considered expired six years past that date. Over time, rubber dries, cracks, and rots, which can be exacerbated by extreme weather. An older tire can suddenly deteriorate while driving, and your whole tread could peel off, causing a serious accident.
Your tires may need replacing before the expiry date because the treads are worn. Your state may even have a law that dictates a minimum tread depth.
Generally, a tire tread shallower than 1/16 inch is considered unsafe or illegal. You should frequently measure your tire tread or have a mechanic do it during routine maintenance. Replacing all your tires at once is a good idea to ensure they wear at the same pace.