I recently bought a bag or two of "Senior/Overweight" food because Jasper developed "love handles" while I was on vacation--the petsitter loved him too much!  But I realized that both he and Chelsea are now 8 and technically "seniors."   Chelsea in particular is still extremely active (Jasper was always much more laid back) and my question is, is "senior" food desirable at this point on the basis of age alone?  (I'm assuming it has less fat and protein.)  Or  is it more of a behavior point--dog slows down a bit.

Or is it necesssary or a good idea to switch at all? 

Appreciate any thoughts.