Along with turnout and a good joint supplement, trimming/shoeing is extremely important as well.
My 1 gelding is on equinyl combo (the intense) they also make an extreme.
He's 9 years old and along with conformation issues, what he was used for in the past contributed to his arthritis (in his right knee and both hocks)
He gets 24/7 turnout when the temp stays above 40 degrees at night, but is stalled overnight when its below 40.
I would give you horse as much turnout as you can.
Also, watch your horse's weight. A heavier horse with arthritis will make it worse than a leaner horse with arthritis.