Our cat is 3 years old right now, and our dog is just a bit over a year old. We thought that when we got our puppy, that it would be nice to be able to go on walks all together as a family, so as well as getting a leash and harness for our dog, we got one for our cat too.
Unfortunately though, it didn't go as well as I would have hoped! They play together really well indoors and do follow each other around, but as soon as we put the harness and leash on our cat, he just plopped down on the ground - completely disinterested and didn't want to go anywhere!
I'm curious though as to whether you'd really have to start leash training them when they were kittens. At the point in time when we got our puppy our cat was already 2 years old, and I think perhaps a little set in his ways already. Also, maybe if the cat was younger than the dog, so that the cat did look up to the dog more, perhaps? Since our dog is the younger one, it's almost like he looks up to our cat, rather than the other way around!