It depends very much on the breed. We currently have a houseful of senior greyhounds, and through my volunteering and our business I meet a lot of very elderly greys, lurchers and Staffies. 10 for any of these breeds is just late middle aged.
In general terms, I'd say don't give up on senior dogs - they can often suffer from expectations that are too low. We had a 12 year old greyhound who did his KC GC Gold Award. Ours all enjoy varied activities, and I'm taking two to classes at the moment - we are doing Rally. At a recent Rally event, I saw a beautiful 14 year old dog doing its test - very inspiring. We also do regular lure coursing, sighthound playgroup, sighthound agility, dog shows, greyhound walks, and ours also have an additional role as supermodels for our business, as well as ambassadorial duties at rescue events.
Health wise - do everything you can to maximise their good health. Pay close attention to diet, environment, weight, preventive health care (worming, vax, teeth etc). Do not skimp on annual visits or general vet care - with oldies (as with pups) they can go downhill much faster if ill, and find it harder to recover, so I always err on the side of caution and where I might not see the vet with a younger dog, will do with a senior. It's so important to have a relationship of trust and respect with your vet - discuss, ask questions, and if they have chronic conditions such as arthritis, make sure that you and your vet in partnership are regularly reviewing the condition and how you are managing it.
Chronic pain is not and should not be part of ageing, and modern vet care can do so much for conditions like arthritis.
Seniors bring a very special gift with them. We do not know how long we'll have with them, but that just encourages us to treat every single day as special and joyful.