Isn't it late for me to learn kickboxing?

Not at all but you are starting to reach the age where your body does not recover as well and as easily so take a gradual approach to things. By that I mean build yourself into shape and you will more easily avoid any training or start-up injuries, aches, and pains. You might even want to do some light stretching and jogging before starting at some school or gym as well as some crunches, squat thrusts, jumping jacks and get yourself into a little better shape before starting. This will help you even more to avoid aches and pains if you have not been very active physically speaking. Also remember that it took some time to get out of shape so it will also take some time to get back into shape.