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.
I inherited a Crossman pellet pistol from my father along with some cartridges .They were about 20 years old when I tried to use them.They worked but only had power for a few shots. Another 20 years later I tried again. This time They did not have enough for even one shot.
SO..They last a long time but Not Forever