Saying no is all part of learning to set your boundaries, and then not feeling guilty about setting them. I think thats something we are constantly learning and adjusting.

As far as diet/exercise habits - sure I think you can change them. I think, I hope, we learn as we get older that we have to change them! When your a teenager, you don't think of long term effects - your immortal at that age. As you get older, hopefully wiser and more disciplined you think about these things. Some people will never change their habits, others are more driven to improve their health and feel better. Again, discipline is a key word - not everyone will want to commit to that level of discipline......