You can harness train your cat at any age, as long as they’re at least eight weeks old. The experience is a little different for kittens vs. adult cats though, so let’s break it down.
Kittens are very adaptable and receptive so they take to training more easily than adults. However, because they are small, signs of resistance or discomfort may be subtle. It's important to pay close attention to their body language to make sure that they're within their comfort zone. Experiences at this age can be formative so it's best to go slowly to build their trust.
Got an adult cat? Unlike old dogs, old cats can learn new tricks. It just may take a little longer. They need more time to adapt to the harness and to the new routine, but with time and patience, adult cats can be trained just as well as kittens.