I went to a smallish middle school and high school. All the kids had known me for years with arthritis (my teachers in elementary school always had me give a talk / report to the class about my disease each year). So it was never an issue for the casual dating I did then. They all knew.
In college, I didn't disclose my arthritis until it seemed like my now-husband was a guy I really wanted to stick with for awhile, even though we didn't know each other for long. I think I disclosed within the first month? His main question was: tell me the worst it can get, so I know that right away. I didn't discuss my arthritis with other dates, because I could tell they weren't going anywhere long term. I never expected things to end up as they did (married 20 years now!), but they did. So there are some young guys out there who aren't thrown by this. My husband now says it was sort of nice to know up front, instead of being blindsided by arthritis after being married 5-10 years.
There use to be a cartoonist / blogger who wrote http://singlegalsguidetora.typepad.com/ and talked about dating and disclosing. It's old but a REALLY GREAT blog. Basically, she disclosed when it got to the point where her date would start seeing more of her, and it would be obvious she was taking meds / resting more than people her age, etc. Sometimes it came up naturally. I remember one time she was talking about how she used to do A LOT of yoga, but was just getting back into it. The guy asked why she stopped, and she disclosed her RA.