This might sound like a trite answer, but here goes...
The only chance you have of being truly successful at kickboxing or any martial art is if you LOVE it, are passionate about it, live, eat, and breathe it.
If you do LOVE it, then you shouldn't worry about what age you start, you should do it for the love.
If you're one of these people who thinks "I could be good at that, I bet I could make a career of that" chances are about 99% that you'll tire quickly of getting beat up in the gym when no one is watching. I learned that the hard way. I didn't "have it mentally" in the sense that I just didn't care enough about fighting to get my ass kicked several times a day, five days a week. This doesn't mean I'm weak, in fact I'm pretty tough, but to become successful in any combat sport you will catch a ton of ass-whoopings along the way. If you don't REALLY want it, a job at McDonalds will look like a great option. If you do REALLY want it, age is unimportant, just do it for the love and see what happens.