Can you teach an old dog new tricks

 So I've spent the last few days looking back and rereading some old blog posts and realizing how much of your life is taken over with life style and bad habits.  As I sit here as I do every night with my dog at the off-leash dog park, I'm watching another dog owner training his dog. He has to follow him, sit down, lay down and come by using treats as  incentive.  Getting into the good habit of doing what your trainer  says but always being rewarded with food. My dog just haphazardly runs around and barks at the gentleman thinking that barking will give him a treat as well.  Making an effort and being trained as a dog when it's a young puppy is the beginning of a lifelong well behaved and well-trained dog.  Similarly, when your child is young, it is good practise to also introduce good and healthy eating habits.  Changing old habits are hard to break, in any case changing my bad eating habits has been the biggest problem in my life, makes it twice as hard to change. Can you or can't you teach an old dog new tricks?