aww

The number one thing you'll be watching for is changes in eating, drinking, and bathroom habits. Super common for older cats to develop kidney issues/failure with age so if they're drinking a lot more than usual or using the bathroom a ton, that's cause for concern. Arthritis also makes it hard to keep the nether regions clean, so UTIs can also pop up from time to time.

As with getting older yourself, be on the lookout for any new lumps, bumps, or growths as they could be cancerous. Especially if they change shape of size.

Try to keep the nails trimmed. Older cats exercise and scratch less so their nails can get too long and become ingrown. It sucks for them and it's a trip to the vet for you.

If they've got longer hair, paying a groomer every once in a while for a trim can be worth it. Long haired cats already get mats more often, but doubly so once they're old.

Be sure you're cleaning their teeth regularly. Brushing sucks for all, but even some finger wipes are better than nothing. Otherwise, you'll be eyeballing an expensive bill for anesthesia to clean dirty teeth and remove the ones that are too far gone.

We're super lucky with him because he's actually really healthy for his age. I think he won the generic lottery or something. He's a very confident and brave cat so it's not often he's stressed. I reckon that also has something to do with it.

Anyway, I hope this helps :)