You don't mention, or I mis-read (and missed it) if this would create serious health issues if you were to try to have a child, or if it would mean you can't take care of children long term due to the issues with weakness and such.

I don't know if I'd trot this out on a first date, but I'd tell people soon. In my case, I love ballroom dancing, especially the tango, and yearn for the day when I'll be doing the tango with that special someone. I'm also an enthusiastic cyclist. In my case, I would want to know soon.

Such a condition would not preclude friendship, and it would not necessarily be a deal breaker in a relationship, but let's face it, if we go out on a date with someone and have a wonderful time, we can't help but imaging sharing some of what we love in life with them. If I knew, from the start, that there would be no kids, no tango, and no cycling, I could handle it much better than if we had been dating a few months and I finally find out that's why you've always had a reason to not go to dance lessons with me.

At that point, I'd be focused on getting to know you, but I'd know that you'd never step onto the dance floor with me to perform or compete, so I wouldn't be getting my hopes up for something that is impossible. If, on the other hand, you waited a good while to tell me, and you let me know AFTER I was emotionally invested, that might be a deal breaker -- not because of the disabilities, but because I'd feel ambushed.

Another thought I haven't seen mentioned is to take the guy's lifestyle into account. If he has a date with you the night before he's going to go rock climbing the next day, then this is going to be a big deal to him. If, on the other hand, he spends a lot of his time gaming at a console and reading and watching movies, then it'll be less of an issue with him. Also, if it effects your ability to have or care for kids, then finding out how a man feels about that will let you know how critical it is to tell him quickly.