It varies a lot between dogs.

My BC is 13 now and only just started to slow down and show her age. Her mother started showing her age when she was younger...about 9.

People often say that little dogs live longer but when you check out the longest lived dogs that made the records, they always seem to be medium sized dogs like collies, cattle dogs etc...
I know two people with medium sized dogs who are 16 years old at the moment and seem to be doing fine.
I also met a bull mastiff who was 11 and she seemed to be doing fine as well. They usually only live to about 7 or 8.

I knew that from 7 onwards, medium and larger breeds are considered senior and many dog insurance companies will not take on dogs that are 8 or over...they'll only carry on insuring dogs that were with them already.

I'd start your dog on glucosamine sulphate and have some cod liver oil in her diet too. I'm sure doing this helped our Jess stay in such good shape for as long as it did.
It's not uncommon for dogs to start stiffening up a little at about 8 years old.