There's no harm in using reward-based training to make 'Sit' into a game from an early age. While a pup is unlikely to learn 'Sit' at 8 weeks, don't be afraid to engage it in simple exercises. All dogs learn at different rates, but it's acknowledged that from around 12 weeks they are capable of learning simple commands. Starting the learning process at this young age helps to establish a mindset which has the dog listening to you and your commands.