When it's time to let them go.
I spoke with my mom over the phone and she told me my cat is going to be put down this Friday. Ivy is 17 and she's had some sort of thyroid issue lately that has been causing her to lose weight, which is really bad because she's always been really scrawny to begin with. Apparently she's all skin and bones at this point and it doesn't look like she's ever going to get any better.
I've had plenty of cats come and go over the years and it's always depressing as fuck when they die. I was in high school when we got Ivy, I pretty much raised this cat.