Puppies will learn at a much faster clip and do not have ingrained behaviors that must be un-learned. Thus, any training regimen is sure to be more successful when conducted with a puppy for a subject instead of an older dog.
If you werent able to start training a particular dog when he or she was younger, dont fret. Training is still effective for older animals. However, if you do have the chance, start training early and youll reap the benefits of increased effectiveness.