Puppies are cute, but they require a lot of work the first six months. With an older dog that period is behind them. An older dog lets you see more what kind of dog you will have. The size is more predictable. The disposition is known.
Older dogs tend to be calmer.
They have frequently been trained to some degree. They are housebroken. This is a matter of personal taste. The Doberman is a shorthaired dog. No elaborate grooming is needed.