it depends on what kind of training and your own beliefs

I start obediance training with my dogs as soon as i get them..

but for stuff like agility, weight pull, flyball, etc. i PERSONALLY believe a dog shouldnt start any serious work/training until about 18 months of age.

Too much jumping and running can seriously mess up a puppy when you do it at a young age, you might not see the damage when the dog is young but it catches up with them when they get older.