Another vote for Premier Flex HA.
Another horse on my yard is in her twenties ex riding school and is on the Premier Flex Plus with good results.
Also palatable
Our Vet physio recommended spillers supple and senior balancer. She isn't one for feeding compound/complete feeds, but said its a good one as has all the right levels of 'stuff' (technical term) in it. My lad is more comfortable on this and boswelia than he has been for a while. But, he is also doing all his exercises the physio gave him religiously every evening - much to his disgust.
Honestly I just don't think it's good enough to chuck a supplement in because you suspect arthiritis. Whatever age, if the horse seems uncomfortable he should see a vet and get a professional opinion and treatment plan. Whether that's medicating, supplements, pain relief.
My horse was diagnosed by a vet (lameness specialist) with fetlock arthritis today (???) and I was actually recommended to start him on a supplement with green lipped mussel. Me being me - ie very suspicious of supplements and claims that they will turn your horse's life around, actually did some scientific digging around and found peer reviewed studies that actually support the claim that green lipped mussel works while at the vets. So I bought a bucket. Now the problem is to get my poor horse to actually eat it. No success so far, I've seriously never seen him so disgusted and offended ??
Apperently atleast the Swedish version of NAF superflex has green lipped mussel in it, so I might try that next.
Oh and by the way, he was injected with steroids today and if it doesn't work vet suggested stem cells or PSGAG next, so we're not solely relying on supplements.