Can you really teach an old dog new tricks? Yes you can, it just takes a little more work training older dogs. There really isn't much difference between training a puppy and training an older dog, you just have to break more bad habits with an older dog.

There are lots of resources available to you so don't try training older dogs without checking around for helpful resources.  There are a lot of tips and tricks that can save you time and your sanity. 

The first 20 weeks of a puppies life is his most valuable learning time. A puppy's brain is like a sponge, soaking up all of the information it can. Training older dogs takes a little longer because their sponge isn't so absorbent.

You also have to deprogram older dogs or teach it how to break old bad habits. This is usually the hard part. There are many products out there that will help your dog break its bad habits.

For example, if you have a dog that barks constantly you can try a special collar that will give it a little electrical jolt every time it barks. If you think that's a little too harsh you can buy a collar that sprays citronella (a scent that dogs don't like) every time they bark. It's more of a puff of citronella air then an actual liquid spray. Since the dog doesn't like the smell it avoids barking.