When did you start senior foods?

Hmm. I always kind of mix what my horse is eating according to his body condition. The basis is I give him good grass hay twice a day. I also give him the Triple Crown feed that is a vitamin/mineral supplement without grain. I feed the recommended amount of that and add 1/2 cup of flax seed year-round. At the beginning of winter I added a cup of oats to that (I'll drop the oats when the grass begins growing in earnest again). At the beginning of January I noticed that he was needing more groceries (no fat over his ribs) so I bought a bale of alfalfa and am supplementing his grass hay with a flake of Alfalfa a day (he could not eat enough of the grass hay to keep up his weight) and added a little more oats. I also noticed near the end of December that, with all the rain and mud he was spending most of his time in the stall and his joints were 'clicking' when he would move around. I purchased and added a glucosamine, chondroitin, HA supplement to his feed.
 
When spring comes and he is on grass I will delete the oats and alfalfa. Once he is acclimated to the grass and is out on it most of the day, he will only get the grass hay in the evening (along with the Triple Crown, flax and glucosamine supplement).
 
I guess my answer to your question is, what does your horse need? My opinion is that our horses need different feed at different times throughout the year. Depending upon the Senior feed, it may be just a different vitamin/mineral supplement or it may be a complete feed for a horse that can no longer chew hay efficiently. There is really a lot of good information out there if you decide to research it. If you have your vet out in the spring to do a yearly health check and vaccinations, ask him or her what their recommendation is.
 
Good luck and congratulations to your horse for having an owner who cares. 

Thanks largely to progress in veterinary medicine and a better understanding of nutrition, horses can remain healthy and sound well into their late teens and early twenties. A twelve-year-old horse is still in his prime.
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Demin Martin

I give my horse a good grass hay twice a day, and some vitamin/minerals supplement. and the horses need a different feed throughout a year and vitamin too.
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Earl Nunes

Last fall had a discussion about this with our vet. Our oldest horse just had his 20th birthday yesterday. We use Nutrena feeds and the vet said that nutritionally there isn't a whole lot of different between Nutrena Safechoice and their Senior feed. He has no problem with eating. The suggestion was if/when he started to have problems, to add water to the Safechoice to make it easier for him. He gets beet pulp and we actually cut that back some as he needed to lose some weight.

All my horses eat SrGlo.  (when they're home)  The filly (almost 4!!) was behind in growth and I was going to put her on a Jr feed but it's nearly the same thing.  I stopped mixing feeds together because someone with a PhD formulated my feed.  I trust it.  They all looked really good.  I do add alfalfa pellets to make it complete (per mfr) for the ones that don't get as much out of the hay.
My horses are (roughly) 14, 18, 22 and 4. []

When an older (teen age) horse starts losing weight, dropping hay out of his mouth, I first make sure his teeth don't need floating and its not worms.  I try  just supplementing his regular diet with some senior (I like Purina senior) and if that doesn't work, then gradually change him over to an all senior diet. 

My oldest gelding is about 17 and he is roly poly..... the only thing I've changed is adding MSM to his feed to help him out in his joints since he still does jumping a little bit.  I get his teeth done annually if not earlier depending on how he's doing as far as his chewing.  He gets grass hay and Strategy pellets, plus flax meal.

I still mix my horse's feeds because mostly I have to watch out that Kabarr doesn't put on too much weight. I don't want to give the 'complete' feeds because, if I have to cut down on the feed, I will also be cutting down on his vitamins and minerals. I just add calories (through alfalfa and/or oats - easy on the oats) as needed. The flax is for Omega 3s.

Firstly, keep a watchful eye on the weight changes or changes in eating habits and see the vet about teeth etc. Also keep an eye on the feet. They also will show changes if being used differently. For example, my old girl was showing knee soreness, I brought the toes back and upped the flax and now its good, but she also has a stifle problem that doesn't affect the wear on her hoof, but the way the hoof flies....its the other foot taking the brunt of her weight that is changing, so I'm starting glucosamine supplements, along with a low Vit C. to help with the stress of aches and pains.
She is also on a balanced diet. Like all the things that horses need, older horses need the same, but a bit more. Their ability to efficiently metabolize nutrients does decrease with age, so the intake needs increase to insure they get what they need....they're starting to match what a colt needs to grow.
I've found that most performance feeds are very similar to the mature horse feeds in nutrients, just that the delivery is easier to chew with the mature one. Sometimes the sugar will be lower on the performance feeds.
Learn to read the labels before you make your choice. The sugar should always be low. That would be the NSC (NFC) value on the label and should not be higher than 16%, 10-12% makes me more comfortable.
The first biggie is adequate protein. My horse is 1100lbs. and her requirements are 700g Protein/day. If she was younger, 500g/day would have met the need, but she's older.
You want to see flax on the label for the omegas. Flax is a perfectly balanced delivery for the horse. If I see the omegas being delivered by way of sunflowers, I pass.
Other things that are related to older horses are: Selenium, Copper, Zinc, B vitamins, including biotin, Lysine, which must be present in order to metabolize the protein and others like threonine, methionine, and leucine which are the key amino acids.
Iron is the bad guy. Most times you won't see iron listed on the label, but if manganese is high (iron's partner in crime) then know the iron is probably high as well.
One thing that comes to mind that happens more often in older horses is colic. I guard against that by feeding 1 tbn. iodized salt/day to insure adequate water intake and slow feeders to deliver the hay all day on a regulated basis. It regulates so well that horse will lose or gain weight if they need it. They also avoid impaction colic and stay happier/busier/warmer longer. I also water down the hay with a watering can religeously.
Learning to read those labels will help you make these decisions in the future. Labels can really vary. Some of them "get it" and some of the don't. Your best defense is to know. If your horse is hard at work, these levels are going to go up accordingly.
Hope this helps!