Razzle, I'm so sorry to hear about Geets.
He has lived such a long life with you, and you are giving him the ultimate gift by ending his suffering.
A younger cat is less likely to have a disease or disability.
This is not always true... and more often than not, when someone with financial troubles gets a young cat solely because they assume
there won't be expensive vet bills... that's when the cat ends up being the exception and having a chronic disease that needs treated, something like irritable bowel disease (which can be expensive to treat/discover and needs a very high quality diet, steroids, etc to prevent flare ups), or other unexpected illnesses like stomatitis, which usually requires an extraction of all of the teeth... very expensive.
I have to agree with Marie here. An animal is not something that should be taken in when finances are a problem... because of all of the unknowns that can happen, and often do happen. Life seems to be that way, when money is the tightest, that's when the animals get sick, the cars break down, etc.
Marcia, our member who is pretty much foster-mom of the century, can tell you firsthand that she has plenty of expenses I'm sure and countless hours/work put into her cats. I knew someone who fostered through a rescue and they had to pay for all of the cages, all supplies, pretty much everything, and had to prepare a room for the fosters. The rescue only provided a particular dry food, so she had to buy the wet food. Sometimes they provided litter, sometimes they didn't. And if an emergency happened during the weekend or on a holiday, she was responsible for the e-vet bills. The rescue only paid if it was during business hours and if it was through the particular vet they worked through. So it's definitely something to prepare for financially at times.