agreed with the rest! The earlier the better. I started Keljin at 4months as well, with a guinea pig mesh harness and that worked great for his kittenhood. It is getting small now so I've gotta start looking for a new one

I would practice putting it on him in the house for 20 min at a time, and he would just lie down and be sooo upset. and when he did try to walk, he would stiffen up his arms so funny and walk like a robot kitty. After a few weeks of practice and increasing it to an hour or so with the jacket, he was maneuvering like a normal kitten inside the house again with jacket (running/jumping, balanced, coordinated) Once I was confident in his comfort with the jacket, then it was time to start taking him outside!