I have to agree with most of the posters here - wait until there's a "reason" for senior. And, ideally, increase hay first.
My mare is going to be 29 in February and she's still not on a senior feed. She has all her teeth, just one very minimally loose one - not loose enough to justify pulling at her last float, and she holds weight well. She simply gets a ration balancer [Triple Crown 30%] for her vitamin/mineral needs and I add soaked beet pulp or alfalfa pellets as needed [soaked simply because she loves soaked feed!].
I have found that her winter protein needs have increased as she's aged, so I feed her a higher protein diet than the average horse her age probably gets/needs in the winter.
I have also noticed that she's been less of an easy keeper this year, but nothing dramatic or terrible. And, at her age, she has every right to be a little more work to keep weight on!
Anyway, bottom line: no reason to switch until you have to.
My mare might never go on a senior feed...we'll see!
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.