that's a myth and easily debunked by a quick google search.
When Dobermans grow older, their skulls stop growing but their brain doesn't. As their brain becomes larger as they grow, it puts pressure on the brain driving the Doberman insane and causing it to “turn” on its owner.
Of course this is ridiculous. Doberman people merely shake their heads in wonderment at the total lunacy of these claims. When they are properly bred, raised, socialized, and trained, Dobermans will love, protect and often die for their families. This loyalty to his family is instinctive. It will develop naturally and you don’t have to do anything to develop it. Of course obedience training is a different matter and should be considered by all Doberman owners.
Just like any other dog, when the skull stops growing, so does the brain. If Dobermans routinely went insane and became vicious, nature would eventually target them for extinction. However, if you abuse any dog, a dog can, and probably should, turn on the person perpetrating the abuse. You would probably do the same if you were abused or threatened. Only a %^$%@*! abuses any animal and those who do, in my humble opinion, deserve what they get.
source - http://spear-barkennels.com/About_Dobermans.php
but there's tons of info on this if you want to look. if this was real it would be happening all the time since there's millions of dobermans out there.