In Canada, most types of boats require Canadian-approved flares less than 4 years old as mandatory equipment. Canadian law also requires that marine signal products be replaced 4 years after date of manufacture (The Canada Shipping Act) - dates are stamped or printed on all flares. Flares under 5 years old should be safe to keep for back-up if stored properly.
Be sure to keep all your signal products dry and protected from heat in a labelled waterproof container or ziploc bag. The chemicals in the flares break down over time and they can misfire and injure anyone nearby. It is illegal to fire a flare if there is no emergency. Keep away from children - they can kill!
Flares are considered to be hazardous material and must be disposed of the same as explosives and fireworks. To help boaters dispose of expired flares in a safe and environmentally responsible manner CPS-ECP and selected CIL Dealers are hosting Disposal Days (link below) where you can bring your outdated flares to be properly disposed of, free of charge.