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I want to suggest that the answer is simply “dont”.
While Sam A has a point that sex and puberty (as well as older age, some dogs do get “grumpy” with age) plays a part in dog aggression it is more likely that it requires a specific incident to trigger aggression. So really, age is not the breaking point. That said, an owner (of breeds with a bad reputation in particular) should always keep a close eye on their dogs interaction with humans and other dogs to step in when it gets too rough from either side. If signs of aggression show it is crucial to be on top of it asap. That can mean addressing and correcting the dogs behaviour right in the situation and practicing the situation later on by oneself or contacting a dog trainer as soon as concerns of a reoccurring issue are raised. Aggression can be a highly ritualized behaviour and therefore must not become a default reaction or ritual in specific situations.
Good socialization at a young age plays a big part in preventing aggression. A dog whos familiar with “normal” life and surroundings and has properly learned to engaged with humans and other dogs is less prone to aggression. Theres simply less to stress about and the dog will not be pushed over the edge as easily as a badly socialized one. Also, just like kids dogs learn and refine their social skills through the interaction with humans and other dogs and should therefore make as many pleasant contacts with them (and other species) as possible at a young age.
In addition, worrying too much about aggression issues can become a true self fulfilling prophecy. Dogs read human body language really well and will notice if you become tense when you meet another dog/human or when your two dogs interact. When you become tense that usually means something is wrong and your dog might adjust to it by copying your state. And a tense dog is always a dog that might react with aggression.
If youre worried about aggression just between your own two dogs you should try not to give them reasons to be rivals. This targets areas such as food (everyone gets their own share at the same time), toys (no fighting about toys unless its in a playful way during play time), cuddles (everyone gets their fair share, no one is excluded unless theres misbehaviour),… You can see where this is going.
Bottom line: Aggression can always happen and socialization and a close eye are your best friends on this topic.
Edit: I just now saw the bounty text. As implied in my answer I dont see any reason why you should separate your dogs while youre at work. And even less an age at which you should start to do so. Separating them can even lead to issues, because the separation would most likely be stressful to them if theyve been kept together before and while youre at home. Its a double-edged sword of course, but as long as your dogs dont display signs of aggression when together I wouldnt see any reason to be concerned.
Have you thought about installing a webcam or similar while youre at work? Being able to frequently check on them while you cant be there might ease your mind.
Inter-dog aggression occurs much more frequently in non-neutered male dogs. Usually signs start appearing when the dog reaches puberty (between six and nine months old) or becomes socially mature at 18 to 36 months- depending on how fast your dog matures. In my experience having one dog desexed and the other entire both of the same sex also increases dog on dog aggression if they are both adults, especially if humans try and keep the desexed dog at a higher rank. As you are raising them together, they shouldnt become aggressive to each other.
I havent heard of dogs that are fine when humans are around and aggressive to each other when alone. Have heard of dogs that fight over their human and are fine alone. So you should see them fighting if there are problems.
I know some owners prefer to wait till their male GSD are 2yo before desexing to ensure that they look as masculine as possible. Aggressive females can become more aggressive when spayed so that is a good reason to get your Presa Canario spayed sooner rather than later to prevent her becoming aggressive.
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Why does my dog display aggression?
Dogs may have inherited an instinct for fight or flight when faced with a threat. The display of aggression in dogs may be genetic (inherited from the dog’s parents) or a breed specific behavior. Certain breeds of dog have been selected and used to guard livestock or alert to territorial threats. Different breeds of dog have been selected to display predatory behavior. The display of aggression may be learned from previous experiences because aggression may have been successful in avoiding or preventing an unpleasant outcome.
The most common presentation of aggression is fear or anxiety motivated.
Some aggression in dogs commonly present at a young age (the first 3 to 4 months of life), yet other types of aggression are more likely to occur during adolescence or social maturity. Aggression in young puppies is most often fear- or anxiety-related. Aggression presenting during adolescence or at social maturity may be sexual, social, or related to territorial threats (which incidentally has a fear component).
What are the warning signs of aggression?
The key to dealing with aggression is recognizing the warning signs that predict a bite might occur.
Aggression is often preceded by fear or conflict appeasing signals, sometimes called calming signals. These signals are offered as communication to diffuse social conflict. Body language that may be displayed in order of the escalation of aggression are the following: