What is the life time of a helmet and how should a helmet be handled?

A helmet, since it is constructed from plastics, i.e. thermoplastics: ABS, Polycarbonate, Polypropelene or thermosets i.e. Fibreglass reinforced plastics, are subject to change from exposure to ultra-violet and infrared rays in sunlight.
A helmet should be replaced every two or three years, However, if a helmet has been involved in an impact, it must be changed without question. DON’T TAKE CHANCES.
Don’t drop it; Keep it out of reach of children. Don’t press the EPS liner inside, don’t carry groceries in it. Clean it regularly.