A dog that may have been rock-solid steady in his housetraining may start eliminating in appropriate places. Now, housebreaking an adult or senior dog again is possible, of course, but the first thing to do is bring him to the vet to rule out (or treat) any underlying medical conditions that may be causing these housebreaking problems, including the possibility of pain. Some conditions that can cause excess thirst (leading to more drinking and more urinating) include Cushings Disease, liver disease, kidney problems, or canine diabetes. Canine cognitive dysfunction ("doggy dementia") could also cause dogs to have lapses in housetraining.
Check your dog for lumps and bumps, particularly any that appear suddenly or that appear to change in size, shape, texture, or color. Check all over his body, including in between his paw pads. Note the location and size of the lumps and contact your vet for advice. Your vet will do a physical exam and ask you questions about your dog's behavior. He or she can then suggest various courses of action.
Many lumps are benign, such as fatty cysts or warts. If you or your vet believes it might be malignant, a fine needle aspirate may be possible (where they suck some cells out of the lump), or the lump may be removed. General anesthesia always carries a risk... so if the lump isn't malignant or bothering the dog in any way, perhaps it isn't worth removing. Ask your vet for advice.
Adjust your dog's food as required. Many commercial dog foods offer a "senior's diet" which is supposed to be specially formulated for the older dog. As dogs age, their metabolisms can change and they could become more prone to obesity if their diets are not adjusted accordingly. Some people prefer to homecook for their dog or feed them raw diets (or freeze-dried raw food). A holistic veterinarian and veterinary nutritionist may be able to help you create a diet that's appropriate for your dog and provides all the nutrients he will need.
Also look for changes in your dog's appetite. Both increases and decreases in appetite can be caused by medical conditions (for example, liver problems in old dogs is one of many reasons why a dog may lose his appetite). You might also find that your old dog is experiencing bouts of diarrhea, constipation, or digestive problems more frequently.
A visit to the vet may be warranted, for example, if your dog's skin or fur becomes very dry or he starts to shed excessively (not merely his normal shedding, but real loss of fur). Brushing your dog regularly will reveal any changes in his skin or coat early. Treating the problem immediately will help to prevent it from getting worse. The longer the problem exists, the more difficult it will be to treat successfully.
This is known as polydipsia/polyurea, and there are could be several medical reasons behind it. A complete check-up and bloodwork can help determine the cause.
If your pet's mobility is on the decrease, make sure you likewise adjust his exercise routine (shorter walks, swimming, etc). Exercise is still important to keep him healthy. Likewise, try to keep him at a healthy weight as it will put less stress on his joints.
If you notice that dog doesn't come on command anymore, starts reacting as if he's startled when someone approaches, or begins bumping into things, take him to the vet for assessment. Dogs usually start to experience a decrease in both vision and hearing as they age. Unfortunately, there's not a lot that can be done other than making changes to help dogs adjust to their decreased vision and hearing (for example, using big gestures to communicate with your dog).
This can include confusion (even to the point where he doesn't recognize members of the family), restlessness, aggression, separation anxiety when none existed before, and other behavioral changes. Sometimes there is an underlying physical reason for behavior changes; it could also be dementia. Your vet will examine your dog to try to determine the cause. Sometimes older dogs will enjoy more 'alone time' snoozing; in this case, crate training can offer the dog a quiet and peaceful place to get away from other pets, children, etc.
Your vet may suggest a senior's blood panel to screen for some of the more common geriatric conditions so that they can be treated, if necessary. Old dog health issues are to be expected, just like when people age, we experience health issues too. But senior dogs make wonderful pets and deserve to live out their remaining years with dignity, as happy and as healthy as possible.