At what age does a mixed-bred dog become naturally protective of it's home?

Also, it's fenced-in yard? One is a Shep/Rotti and the other is a Shep/Hound. Both are about two years old, and so far, are very friendly. Will they ever become protective, since they are already two years old and show no signs of aggression?
Update: I want people to think the dogs are aggressive, whether they are or not, just by barking at them, at least.
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