Thought I'd update this thread with a recent experience I had disposing of old flares. I'm specifically speaking of aerial flares here, since handhelds can be safely lit off in suburban areas without issue as long as handled responsibly and legally.
Over the years and multiple boats I've accumulated a significant number of expired flares, some of them dating back to the '80s. And as many have said, they're not easy to get rid of. Nobody seems to be willing to take them for disposal and you can't shoot them for practice anywhere on the water. At least not legally. So I rang in the New Year by shooting them off in the mountains in a situation that couldn't cause any issues. What I mainly wanted to address was the results of firing older, expired flares.
Turns out, there's a reason they have an expiration date. While I found flares from my sample group that were less than five years past expiration functioned properly, once they got to around eight years they started having sporadic problems. While a few functioned normally most either failed to ignite or underperformed on altitude, and as the dates got older the reliability got much worse to the point where some were complete duds.
My sample included 12ga and 25MM meteor flares from Olin/Orion and Kilgore. Both the short 12ga. and the newer long case ones as well as metal and plastic case 25MM. There were two 25MM parachute flares but I only was able to fire one. It underperformed on altitude. And since almost the entire cartridge gets launched from a launcher with a relatively small grip it was a little rough on the wrist. Probably comparable to a .357Mag. which is why it's not a bad idea to get some hands-on practice in non-distress situations. For someone with little or no experience with handguns it could be a significant problem in a crisis.
So, my conclusion is that firing old flares for practice, experience and disposal is a good idea if done properly, especially when they've gone well past their freshness date. While this unscientific test wouldn't suggest that keeping recently expired flares onboard is a bad idea, you'll want to check and make sure they're not too stale before you rely on them to save your hide.
The remnants: