Question: I mainly use my road bike during the triathlon season and now that the season is coming back around again, I am finding some old, unused CO2 cartridges from last season (fall). 

I typically use 16 gram CO2 cartridges and they usually fill my bike *to about* 90 psi.

Are these cartridges still safe to use after months of inactivity?

I've tried researching the matter online but all I've read applies to airsoft, etc. and not necessarily the 16g bicycling cartridges I'm concerned about. But, from what I did read CO2 cartridges have a *chance of leaking* if not stored correcty and may indeed have a shelf life.

Answer: 
They last pretty much forever provided they haven't been damaged (e.g. punctured or rusted or something), so go ahead and use them. 

If you really want to check them, you can buy a few new ones and weigh them on a scale (and compare the weights of the old cartridges to the new ones). If they're significantly lighter, they've leaked and throw them out. Else, go ahead and use them. But a few years (say less than a decade) is pretty much nothing in terms of having issues with the cartridges leaking.
Yesterday I was out biking and got a flat. My CO2 canisters are many years old, I’m going to guess about 5 to 10 years old. When I went to fill up the flat absolutely no aire came out. The canister was undamaged and any visible way. In hindsight was quite light. So yeah after many years your CO2 canisters can apparently totally let you down. I was calling for ride. It makes sense. PHI extinguishers for example do lose pressure over years and this is why they are inspected and expire. Believe it or not air molecules can move through the metal wall just happens at a very slow rate. In fact that’s the principle by which your bike tire loses pressure overtime. And easily seen in a birthday balloon On a short time scale