Generally speaking, dogs are fully able to wag their tails around 3 weeks old, but there are also some that are later, and can be delayed until 2 months without wagging their tails. This is mainly because they are more obsessed with food and sleep than adult dogs. When they slowly develop a sense of vigilance, the tail will become a tool for them to convey information. This is why puppies usually wag their tails crazily when they are eating and grabbing food, but when they are alone. It won't shake.