I don't know about percentages. However, taking more than 65 hours to get a PPL is normal; I think the national average is around 60 hours.
I haven't actually heard of ANYONE who couldn't learn to fly. I've heard of several who had an instructor who told them they never would; they went to another instructor and got PPLs, and I know THREE like that who are now instructors themselves. Many who find it that hard run out of either money or motivation or both. But as for physically not being able to learn? I suppose people like that must exist, but there can't be many.
I don't think 'd ever tell anyone they couldn't learn due to their difficulties in grasping the mechanics of flying. But I might if their attitude seemed all wrong - no idea of airmanship, irresponsible, hopelessly immature, something like that; and seeming unable or unwilling to change. Such a person would be dangerous. But it hasn't happened yet, for me.