system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do car seats have expiration dates?
gpt
Material fatigue.  Plastics have shelf lives, and manufacturers can't guarantee efficacy once the materials get brittle or otherwise unreliable. 
EPS foam helmets (like for bicycling) have the same problem.  Keep a helmet for 5 years or less.