Thanks to advances in veterinary medicine our pets are now living longer than they ever have before.  It is important to keep in mind that senior pets can be affected by a wide variety of conditions including (but not limited to): weight and mobility issues, kidney disease, cancer, diabetes, thyroid disease, and heart disease. 
     The age when our pets are considered 'senior' vary depending on the size of the pet.  In general smaller breeds of dogs live longer and are considered to be 'senior' at a later age than larger breeds.  For example a Great Dane would be considered a senior pet when 5-6 years old, while a Cocker Spaniel wouln't be considered a senior until 9-10 years of age. 
     Because of their risk for developing issues it is recommended that senior pets receive semi-annual (twice a year) wellness exams by their veterinarians.  During this visit they should have a physical exam as well as laboratory work (bloodwork, urinalysis, etc..) preformed.  This can help us find developing issues, sometimes even before they become a problem. 
     You may notice that your older pet seems to be slowing down in general.  Often this can be caused by dulling/aging of the general senses (sight, hearing, taste, touch, and smell).  This dulling of the senses causes a slower response to external stimuli.  You can help your senior pet to fight these signs of aging by continuing to play interactive games with your pet and also continuing to work on training commands. 
     The following link will take you to the American Veterinary Medical Association's (AVMA) website on frequently asked questions regarding senior pet care.  Also, there are several links to pages with more senior pet care information on the bottom of the AVMA's page.
     www.avma.org/animal_health/care_older_pet_faq.asp