Old microwaves are as safe as any other appliance, assuming they don’t show any signs of physical damage. If that’s the case, I highly recommend buying a new one, or finding a qualified tradesperson to inspect it. It’s much more likely that the magnetron inside the microwave will have worn down. This is the component that generates the microwave's energy, and they don’t last forever. If the microwave’s seen a lot of use, you may well find that your food isn’t being heated properly.