The Baby Dragon is not hostile. You should make him your pet instead. Though he's kind of annoying and catches everything on fire. Don't bother trying to kill him unless you are waiting for your wheat to grow. It takes forever. He should also be able to keep up with you when you're flying around in creative mode. Great little pal to have around, unless you don't like fire...
Will follow you around and attack most hostile mobs, and will eventually turn into a large dragon that you can fly! Also can be made to "sit" by right-clicking it so its tail stops wagging.
How to kill
If, for some reason (possibly related to mental illness), you want to kill a baby dragon, confine it in a small space and stab it in the face with a sword until it dies. Once it dies, drop an anvil on your head to repent your sins.
Raw beef to tame. Dead bush to un-tame. If you tame a baby dragon and he grows up than you have yourself a tamed dragon! The aging time is random, but when/if he does, you will even be able to ride the dragon. If you really, really,really miss your baby dragon, try right clicking your grown dragon with a diamond. Poof. He will un-grow! And yes, a baby dragon that has un-grown can grow up instantly if right clicked with another diamond.