My two are raw fed, which is obviously different to being fed kibble or wet processed food. But I don't change diet as they grow older. My 13 years 8 months old William is still fed the same as he was when younger and thrives on it.
It used to be thought that older dogs needed a lower protein diet to help their aging kidneys, in the belief that less protein would put less strain on the kidneys. However, in recent years that attitude has begun to change, with the realisation that older dogs may actually need more protein because their system may not process it quite as well, but that it needs to be good quality protein that can be easily processed by the body.