At what age is best for a Dog + Cat to live together?

Puppies and kittens raised together will be well-bonded friends. But a puppy with an adult cat is the next best pair, as opposed to adult dog and kitten, which could be risky. The cat will swat the puppy when he gets too rough, and the puppy will grown up respecting the cat. If the collie tries to herd the cat, the cat will have to put the puppy in his place. Since most cats in "only cat" homes tend to be overweight, the puppy may be good exercise for the cat.