Provided your child can hold onto a bat and understand not to break it (or anything else with it), you can start teaching him or her about ping-pong. Bouncing the ball to them on a hard floor and getting them to hit it around is all good for general coordination.
In terms of when to start kids on a ping-pong table, I'd say to wait until they are tall enough to see over the top of it easily. If they are too small for this, you could probably get an old table and cut the legs down to a smaller size, so they can play at a comfortable height. If you can cut a table down to size in this manner, you could probably have kids hitting balls around as early as 4 or 5 years. That's a pretty early start!

