As long as you buy a good helmet and take care of it, does it last forever?
It may last forever, but that doesn't mean it'll provide protection in a crash forever. In fact, helmet manufacturers recommend replacing helmets every five years. And, it's not so that they can sell a bunch of new ones. It's because the materials in a helmet break down over that time causing the helmet to lose some of it's impact-absorbing ability, which means in a crash, you're at risk. So, the helmet may look dandy, but you shouldn't wear it when cycling.
Something else to consider is that helmets are continually improving. Today's designs are drastically superior to those we were wearing just a few years ago. Desirable features such as better cooling vents, baseball-cap-like comfort, feathery weight, along with advances in protection against concussion and abrasions, make it worthwhile to upgrade regularly.
We stock the latest helmets and can demonstrate the new features. We can also take a look at your old lid to see if it's still ride-ready or in line for retirement.