Why start early
Puppies are learning what the world means. Searching for food gives them a happy way to explore new floors, new rooms, and new objects. They learn that sniffing is safe and that thinking pays.
Scent games are also kind to young bodies. Puppies should not do a lot of high-impact running or jumping while their bones and joints are growing. Ask your vet what level of exercise suits your puppy's breed and size. A box search on the carpet adds mental work with little strain.
It also teaches a quiet skill that many puppies lack: how to settle down and focus. This is a good partner to the basics in nose work at home.