William will be 8 years old in May. He's raw fed, not kibble, and is a very active and slim dog. The advice in the past always used to be to consider your dog as senior when it reached 7 (which is silly as a giant breed will be old at 7 and a toy barely middle aged generally speaking). I've just started to do some online research about it. My main concern was protein levels because a lower protein food used to be recommended for older dogs.
It seems that research and various trials have shown that lower protein food isn't seen as necessary for older dogs (or even those with kidney problems) any more. The quality of the protein is seen as more important. That's as far as I've got at the moment. I know not to believe everything you read online but I kept coming up with the same advice about protein from a number of sources.
William has Yumove tablets to help his joints and Dorwest Keeper's Mix as a general conditioner. My personal opinion at the moment is not to change much with him while he remains fit and active. I may change him back to kibble in the mornings and raw in the evening, but that's only because he tends to get constipated on raw because of the bone content, nothing to do with age. I've ordered some Fish4Dogs sardine kibble to try him on for one meal a day. I chose that one because it's high in oily fish and should help with joint care.
Finding the right food is definitely a minefield, so many opinions. As Pepsi has been a little bit overweight for some time you might want to consider changing her food anyway, but think of options rather than just a senior kibble. Good luck