Purely anecdotal and Her Mileage May Vary --
I'm now 64, female, and have had osteo-arthritis issues for the last 8 years or so. The arthritis in my thumb joints and fingers was particularly bothersome in 2007. Midway during that year, I went on the Zone program and arthritis pain was almost completely gone in about a month.
Fell off that wagon in early 2008 and arthritis issues returned, as bad as before.
Started Primal in 2011 -- still there, with a couple of longer lases (more than a month of too much fruit, a bread binge or two and one memorable ice cream orgy). As long as I stay within the program the arthritis is a non-issue. Benefits show up in a week or less, full relief within a month of clean eating. I have to do a lot of handwriting for my job (primarily document signing, sigh) so I am particularly alert to when the hands hurt.
I don't have to be in full-blown ketosis to get the beneficial effects, but being in ketosis helps so many other aging issues (muscle inflammation, soreness, sleep disruption) that I try to visit keto-land as often as possible.
He not busy bein' born is busy dyin' ~ Bob Dylan