There is no one “right” answer to this question. Just as people age differently, so do dogs. What is ‘senior’ in one dog might still be ‘middle aged’ in another dog of the same years. Do pay attention though, and see if their eyes seem a bit cloudy or if they cannot hear you as well as they used to. Of course, changes in cognitive function are worth noting and if they seem to forget the ‘rules’ of the house suddenly, that could be an indication of a change in their thought process.
How you care for your fur baby may need to change and it is best if you do not get too stressed out. Accept that they are going to be a bit of a different dog from now on and keep an open mind. If you need to get support, see if your local shelter or canine groups offer a place where you can get help and talk to other dog parents to see if they have advice. You are not alone, so reach out if you need to!