I think it depends on whether the cat has any particular problems that senior food would benefit. Jaffa is 10 and he does not have any kidney disease, but his values for urea and creatinine are at the top end of the normal range. I feed him a mixture of adult/all lifestages and senior food. I'm not too keen on senior food in general as often it is combined with a light food, which is fine if the cat needs to lose weight but not if the cat doesn't (Jaffa doesn't). Some of the ones I've looked at contain, imo, inferior ingredients to the adult version, sometimes having a higher carb content. They also sometimes have lower protein, and although that can be good for cats with crf I think good quality protein is more important for a cat without kidney problems or in very early crf. So I'd rather feed an adult food with a good quality protein source. They also usually have a lower phosphorus content than the adult version which can be good for older cats (esp those with early crf), but that generally means lower than the adult version of the same brand, and not necessarily lower than an adult or all lifestages food of a different brand. For Jaffa, I keep an eye on the phosphorus content and try to feed him adult foods with a lowish phosphorus content and I give him senior felix or perfect fit now and then. If his kidney values go up then I might revise his current diet.
So I think it really depends on your cat's individual needs. I wouldn't automatically feed a senior food to a cat once they get to about 8 or so (as recommended on the packaging). Feed the cat not the age! If a cat has no medical problems I'd continue to feed adult food but keep an eye on the phosphorus content as they get older, as it can vary a lot from food to food.
I think the most important nutrient for older cats is water, so I think it's worth adding or increasing some wet food into the diet of a senior cat and/or paying attention to providing water in lots of different ways (different containers, several places, water fountains etc).
