I think it depends what it is. Diabetes, you might mention if they suggest a date at a sweet factory, but generally I wouldn't at the outset. But I met a very wonderful man on OLD, and he had to tell me he's blind, because it limits him a bit in that he can't drive and there are some activities he can't do. But he said he'd chatted to some others online, and some weren't prepared to meet at all because of it. Their loss.
I've also had a couple of men who mentioned their medication at the point we were in bed, as it affected their erectile function, and I'd have preferred to know that before, because they wouldn't have been getting a shag if I'd known (and it's obviously why they hadn't mentioned it.) It's one thing for a long term partner to end up with issues, but I don't want to start a relationship with it, and there are a fair few men in their 60s and 60s that suffer (certainly not all though!)
So if it affects you now, you probably need to mention it out of necessity, especially if it affects accessibility and where and how you meet. If it is likely to affect you in the future, then maybe third date? I've had several dates that only lasted one date, a couple i gave a bit more chance for a second date, but they're only likely to get a third date if I see some potential.
Stuff like high blood pressure or cholesterol, that will be more and more people as you get older, and I wouldn't mention it until it's when you stay over and have to take your daily meds. If it's medicated and you keep active, it probably wonderful an issue.