75 isn't MR, actually, but borderline. At 75 all you'd expect to see is somebody who has a hard time at school.
And yes, of course they know, in all but the most extreme cases, or when parents leave them deliberately ignorant and they're left in a very similar state to some of us were before our parents revealed that we were autistic--rejected by others, feeling inferior and not able to understand why. (And that is just plain mean of said parents. People deserve to know. Especially when they are going to be told in the worst possible way if they don't learn it from somebody who cares--probably being called "ret*d" in the schoolyard.)
Have you yet got the chance to actually meet people with cognitive disabilities? They are different, but it's not even immediately apparent in the mild range, and when it's obvious, they're not walking stereotypes or anything, just people who are a bit weird, kind of like Aspies are a bit weird but still perfectly good humans.