My male started alerting to people on the property at about 4 months old ( a week or so after I got him ) ..but I also have two other large dogs who are VERY protective of the property and us so he may have just picked up on their cues. Now at 10 months he is definitely aware of his property and people. He barks , and is anxious to check out people , see what they are all about.
Alvin - GSD - born @ late February 2013
Bo - Boxer/Hound - born @ late June 2008
Greta - English Mastiff - born somewhere between 2003 and 2005 ,estimated.
ALL MY RESCUES!