I have two labs, one is 12 and the other one is almost 2. The 12 year old's eyes are starting to go, but when she was younger she would let us know when someone she didn't know was coming up to the house. I wouldn't have trusted her to go attack anyone because she wasn't trained for that, but very few people got to the house without us knowing about it. She still tries, but she's getting old and her eyes aren't what they used to be. A few years ago she started barking when family members were just coming up the driveway (about 50 ft from the house) and would drop her tail between her legs when we got close and she could make us out. The two year old, on the other hand, barks every time a flee farts. We don't trust her nearly as much as the other one for alerting us to people, but she'll let us know when dear walk across the back yard 
