Dog being protective

What age will a dog start barking at people hu walks by my house (being protective)

There isn't an age, it is just a behaviour that it develops... its not really desireable, having the dog go mad barking everytime someone walks by... annoying don't you think?
Theres a difference between being protective and barking at people.

It's not an age thing, it's a dominance thing. When the dog thinks that the house and surrounding area is his/her's they're going to defend it.

Different breeds have different levels of being watch dogs (dogs that bark, tho not attack)...This is not so much protective as territorial...They kind of practice at it when young, but as they mature gain confidence (12 to 18 months), they seriously let you know that someone is coming.

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