I have tried in the past, I would love to continue but am having a hard time finding treats mine like that are not absolutely disgusting ingredient wise, but I doubt I'll have such luck so I might have to feed nasty treats, oh well.
It's fun to do but a suggestion: The first thing you teach becomes a very strong behavior, teaching something like "sit" is not advisable because if you ever want to move on to training things where they must move or stand it is difficult, they will repeatedly sit while you are trying to get them to do something else. IMO the first behavior all dogs and cats should be taught is "touch" to poke your hand or an object (a touch stick is ideal, this is any long object that the cat pokes the end of like a chop stick or drum stick) of your choice with their nose.
I tried doing the clicker training with cats at the shelter but they were free fed dry food so didn't really have much interest in my treats, it was too bad considering this shelter was full of semi-ferals that this would have benefited heavily.
have you ever clicker trained a dog (or any animal) before?
Here are a lot of videos showing how, several species are shown: http://www.clickertraining.tv/search.ht ... ords=video
Important things to remember with cats is that they tend to bore more quickly than most dogs so keeping sessions short is a must, you want to leave them wanting more not glad it's finally over.
I shut one of mine in a room while I train the other and it works somewhat, the only problem is the one shut away meows making the one I'm training run to the door and fight with the other cat underneath
Eventually he finishes and we can continue. Some people train their second cat to sit on a mat (like a door mat) while the other cat is being trained. One cat gets clicks and treat for the training session and the other gets clicks and treats for staying o the mat. Of course the "go to mat" command must already be known for this to be successful.
Good luck with it, there is a clicker training yahoo group for cats you might want to check out too, maybe you'll get some ideas. I'd love to see videos of what you accomplish.
Oh and to answer the question of age, you can start right away, if they are old enough to live with you they are old enough to learn. Just remember to keep sessions nice and short and joyful.