In a word : YES --- It can be done. My cat, Hank was leash trained when he was about 3 months old. I got him a special cat harness to go with his leash, that part seems to be pretty important. I don't think it would have worked if I'd just clipped a leash to a collar. The harness fit him around the neck and the chest, going under his arms and so any tugging on the leash would be felt under his arms more than his neck. Also, the harness can be worn by the cat (kitten) in the house before the leash is introduced. Hank used to love his halter and leash but mostly his halter. He used to walk around the apartment with an important strut when he had it on, like he was "Safety Cat" or something, He didn't even want it off after we would come back inside.
I think that he got used to the whole idea of a leash so quickly because it was associated with something that he anticipated with great excitement and enjoyment. From the moment he figured out that clipping on that leash meant he was going to get to go outside, When we were outside I didn't walk him around like you would a dog, but I would stand and just let him explore the length of his leash and then lead me where he was interested in going. To an extent we led each other. Sometimes I would direct where we were going with a tug while maintaining a short leash, or other times he just wandered and I followed while I let the leash out long' If I ever lost hold of the leash, Hank was so attached to me and the safety of being with me at the time, that he would stop and wait for me to retrieve the leash before going forward again. As he got more used to the outside I would let him off the leash for short periods of time but always in seeing distance.
Since then we moved to a home with our own yard and Hank goes outside now without a leash. Sometimes I regret not keeping him up with being on a leash as now he's not as keen to use one, though I have on a couple of occasions, but only because of perceived danger. So, yes, you can train a cat to use a leash. Hank used one until he was two years old and it was great. The best was when people would see me standing with the leash and not see Hank. Then they'd suddenly spot him but they'd be expecting a dog, not a little orange cat, the look of surprise never failed to crack me up.
