The same way you train a younger dog, but, with more patience, persistence, time and always be consistent. At 8 yrs old they have habits that need to be worked thru. It can be done. I had a guy who had a 12 yr old lab in one of my classes. Everything turned out great, he just needed to come to every class and listen to what I told him to do, and I had to listen to his problems and concerns and fix what I could. Get into a good class that teaches positive reinforcement training. Good Luck..
LOL...with great difficulty........remember the saying "you can't teach an old dog new tricks." It's ALMOST true but as drgnrdr says...with patience it might be possible.