When a dog is considered old is very obviously dependent on their life expectancy, which tends to be related to their size. Larger dogs tends to have a shorter life expectancy, whereas smaller dogs have a longer life expectancy. In general larger dogs are considered 'senior' at 5-6 and smaller ones at 7-8.
As dogs get older, like us, they can become less and less active. This means that they are burning less calories, so need to consume less to avoid putting on weight. Older dogs tend to develop mobility issues like arthritis, which cause pain and inflammation around the joints. Any additional weight that they put on these joints compounds the problem and exponentially affects their ability to move. One of the easiest ways to help control their weight is with a high quality food, like Pooch & Mutt's Senior Superfood (or Slim & Slender, if they are carrying extra weight).
There are lots of natural active ingredients, such as omega 3, glucosamine, and chondroitin that can help protect your dog's joints and aid their mobility. Adding these to your dog's diet is a quick and easy way to help your dog stay happy and healthy. The easiest way to add them in is with a mobility supplement, like Mobile Bones, which can be added to any feed. Alternatively there is food like Joint Care, which not only has the supplements 'built-in', the majority of the food is salmon, which is one of nature's richest sources of Omega 3s.
As dogs get older it is harder and harder for them to exercise. Hydrotherapy, where your dog exercises in a pool, is a great way to keep your dog active, without putting pressure on the joints. This does not just give your dog the opportunity to do some exercise, it also gets the joints moving which gets the synovial fluid moving around the joints, helping the joints to regenerate and get healthier. A quick google search will lead you to your local hydrotherapy centre, who can tell you more about it.
Non-steroidal anti-inflammatory drugs (NSAIDs) such as Metacam tend to be the first port of call for a lot of vets. However there has been quite a considerable backlash against them in recent years. Firstly they do nothing to actually help with the problem, they just help with the pain caused by the problem. It is much better to look at the problem itself. Secondly they have considerable side effects ranging from vomiting and tar-like stools to serious kidney issues and even death. We tend to think it can be OK to give NSAIDs at a very late stage in a dog's life, as there may not be long enough for side effects to develop, but with any dog under 10-12 they should be avoided where possible. Our suggestion is that you try a joint supplement first, and only used NSAIDs as a last resort.