The best (and most serious) guardians we've owned started guarding later in age, like 1.5 - 2 years old.
I've had guardians that "guard" at as early as 8 weeks old but grow into fearful nut cases.
I've also seen CO puppies guard (the whelping box) as early as 4 weeks but they will usually stop guarding around 7 weeks and not guard again until 1-2 years... Those pups usually turn out to be the most hectic, confident, and serious guardians I've ever met.
Luytiy is the most successful and predictable guard we own and he didn't start guarding our yard till he was over 2 years old - and he is a VERY serious dog now.
Point is, I think it's best when they mature slowly into a guardian role (gives more time for socialization, which helps build better judgment). In a perfect world, they'd all wait until social maturity to guard (tho with some mastiffs that would be a looooooooooong wait! lol).
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