I want to suggest that the answer is simply "don't".
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 dog's 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 dog's 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 who's familiar with "normal" life and surroundings and has properly learned to engaged with humans and other dogs is less prone to aggression. There's 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 you're 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 it's in a playful way during play time), cuddles (everyone gets their fair share, no one is excluded unless there's 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 don't see any reason why you should separate your dogs while you're 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 they've been kept together before and while you're at home. It's a double-edged sword of course, but as long as your dogs don't display signs of aggression when together I wouldn't see any reason to be concerned.
Have you thought about installing a webcam or similar while you're at work? Being able to frequently check on them while you can't be there might ease your mind.