There's no certain age. He may never grow into it, and I don't recommend training him to do so unless you plan to spend the thousands of dollars, and the mass amounts of time, to professionally train him. Dogs aren't security systems, and there's a chance he'll never grow into being protective. Generally ABs are, but there's a good chance he wont be unless you got him from a breeder that breeds for this specific type of work. If there's crime, etc.. in your area, call ADT. Even if he DOES grow into being protective, which probably wont be any time soon, you should NEVER put the safety of your home, your family, your children, or yourself, in the 100% trust of a dog. Period.
Also, just a friendly word to the wise, asking this multiple times will likely get you reported, and it will generally only give you the same answers.
ETA: Annoying?? How am I being annoying? Wasn't saying that *I* would report you, or that *I* have a problem with you asking multiple times, but there ARE people on here that WILL report you for asking the same question multiple times. So much for trying to give a friendly bit of information to the newcomer. I'll just go back to being a ***** to everyone that thinks their dog will protect them.