The age of 2 is considered to be when dogs will start to have behavior issues like aggression if they are going to develop them. IF. And I've heard of "leash aggression" from others...including here on the board...but haven't experienced it. (my last dog didn't appreciate dogs coming up to him but the leash wasn't really a factor...his did start around the age of 2....and his wasn't truly aggressive...he didn't mind the other dogs as long as they weren't approaching him)
Maybe do a search here under leash aggression while you're waiting for responses. Good luck with your pup.