My first Cocker, Josh, reached the incredible age of 16 years and 51 weeks! I'd so hoped he'd make his 17th birthday, but it wasn't to be. He just died of old age, not of any particular illness.
One of my current two, William, is 12 years and 9 months. He's quite fit and active, although almost totally deaf. I think the main difference between Josh at 12 and William at 12 is that I've made very sure that William has stayed slim, whereas I remember Josh as being a bit portly at that age.
William has Dry Eye and cataracts, but his eyesight is still OK and the cataracts haven't changed for several years and are unlikely to do so now. He has some damaged discs in his back and they're one of the reasons I make sure he stays slim. He has acupuncture every couple of months, which helps, and I add golden paste and a green lipped mussel supplement to his food every day to help with his mobility and his general health. He's raw fed and I find that helps keep him slim and fit too.
He still walks in the New Forest every day and enjoys his walks, although I've noticed that he trots rather than runs nowadays and I know he's lost a bit of muscle tone in his back legs. I lift him in and out of the car, because of his back, and rarely let him go upstairs for the same reason.
I think keeping them mentally stimulated and interested in life is as important for older dogs as it is for older people. So I make a point of playing with him and his favourite cuddly toys for a few minutes every day, vary his walks by going somewhere different a couple of times a week, take him up to my friend's house so he can see his best friend Ruby Retriever (who is even older at 15) and when we're out walking we play games such as finding hidden treats and practice a few sit stays and recalls.
I haven't changed his diet. He's raw fed, which he loves, and I find it helps him to stay slim without me having to reduce or change his food. Until recently the belief for many years was that older dogs needed less protein in case it put too much pressure on their kidneys. But now it's being realised that unless they have a specific kidney problem they need as much protein as younger dogs because they're less able to assimilate protein and may become protein deficient if their diet is low protein. There's also a recognition that good quality protein that's easily digested, such as from meat and fish, is better than protein from grains or other sources.
So overall, I think good diet, appropriate exercise and mental stimulation are all important to help keep them as fit and happy as possible.