There are plenty of reasons a female cat may have reached adulthood without getting spayed. It's often because the cat has been used to produce a couple of litters, or it could simply be that they were a stray before being picked up by an animal shelter. In any case, it's well worth having an older cat spayed because the likelihood of certain types of cancer, especially breast cancer, rises and the risks associated with pregnancy become more serious.
It's generally fine to spay an older cat, but there are some things you should know about.
If you've ever raised a feline from kitten to full-grown cat, you'll know how much their personalities can change; as such, the changes associated with spaying tend to go unnoticed.
If you spay an older cat, it's unlikely that their personalities will change greatly, but the way they behave at certain times of year probably will. The is great since many of those changes will eliminate or drastically reduce problem behaviours, such as excessive yowling, spraying or an overpowering need to escape your home during the heat cycle.
However, it's worth remembering that not all adult cats will cease these behaviours if you get them spayed. After all, they will have become something of a habit, so you shouldn't necessarily expect your cat to act as they would if they had been spayed as a kitten.
Your vet may request blood work before they spay your older cat — this is just to check that all parts of the body, particularly the kidneys, are up to the procedure. It's unlikely that anything will be wrong, but it's worth remembering that your cat's recovery process will take a little longer than it would for a kitten.
This often surprises people who have had kittens spayed in the past; they usually bounce back, showing few signs of discomfort. This is partly because of their youthful energy and partly because of the fact that their immune systems are a lot stronger. In contrast, adult cats won't have as much energy and their immune systems will be weaker. Your cat may be lethargic, unresponsive and refuse food for a couple of days if you go through desexing when they are older, and they may not be back to their normal selves for a week or two.
It's worth underlining once again that your cat should be fine after being spayed, but its weakened immune system is going to make infection slightly more likely. Make sure you watch out for any signs of infection and contact your vet if you think anything is seriously wrong. The desexing procedure is a lot tougher for female cats, so you don't want to take any chances.