As your dog ages, there will certainly be some differences in their behavior. Some things will be perfectly normal and to be expected and some are not normal but can certainly come up. It is highly likely that you will need to take your canine family member to the vet to be sure what you are dealing with and to take care of it as soon as possible. With aging dogs, it is better to take them to the vet and have it turn out to be nothing, than to wait and allow the problem to become worse.
Regular checkups and preventative visits are always recommended, anyway, but if you notice some of these things, you really should get your dog into the vet right away: hair loss, itchy skin, changes in their urinating and/or drinking habits, changes in weight and/or eating habits, bleeding gums, excessive bad breath and of course any changes in their ability to hear or see or walk etc.
In general, though, you may just be concerned about helping to keep your pet as comfortable and happy as possible. Here are some suggestions from About.com that will help you get a jump on keeping your friend happy and healthy.
Provide a comfortable clean bed – there are many “orthopedic” beds (….) available now at pet supply stores to ensure comfortable rest.
Provide fresh water daily (note changes in water consumption) and a healthy, age-appropriate diet.
Don’t expect too much from your senior dog – s/he may want to run and play Frisbee like the good ‘ol times, but go slow — heat, arthritis, age-related muscle atrophy, and other age-related effects can take their toll.
Ease distractions – senior pets can be easily startled by or become fearful of: kids, loud noises, and general commotion as they age. Conditions such as arthritis can make the dog fearful of getting hurt (or the potential to get hurt) with sudden movements of kids or being stepped on. (Full article here)
Your senior dog may sleep a little more and require a little more TLC, but that is okay. Just take it easy and be patient with your pooch and yourself! Keep your eyes and ears open and you will be ready to react just in case anything might come up. There is a good chance nothing bad will happen and your dog will simply get a little slower and have a bit less pep than they used to, but know that your love will help carry them through, no matter what.