If I find a bottle of ejuice that's very old, (like plus 3 years) I will toss it. That's gotten a lot easier to just *do* now that I am DIY.
That said, I've vaped juice that was a couple years old, just out of curiosity. Probably not the best plan, I was still dual using and my tastebuds? Not like they are now. Juice turning darker isn't necessarily *bad*, but I prefer to use it if it's in a glass bottle just out of worries about plastic leeching or something (despite my clumsiness, I am still a glass bottle kinda gal, though those tiny, squeezy bottles with the needle nose tip are way easier for filling.) Normally I don't take that much issue with plastic, but there ARE a lot of possible things in e-juice I don't know about so it just seems safest and I use dark glass bottles anyway, so I only see what my juice looks like as I'm dropping it in there and I have a feeling I will be having less "old" ejuice laying around in any case....
I guess taste/vape would be the way to tell, but there's really not much "scientific" data on that that I know of. I did ask a vendor once and they only said, "Our juice improves with age," which may be true... to a point.
So I'm not sure how helpful it was....
I've never gotten sick from e-juice that I *know* of, fortunately the older flavors I was buying back in the day were all tobacco flavors, which I now hate, so it's even easier to toss those bottles....
Anna