I won't try to come up with numbers because I don't think I can. However, it seems like a lot of older ones WANT to believe very, very badly. Some may fully believe...others, it's like they believe to some degree and try to believe the rest. As others mentioned, they are very afraid to leave all that they have ever known.
For younger ones...I don't know how many can really truly believe.
But it is a cycle. As the young ones grow up, and have given so much of their lives to the truth, they need to believe in its truth more and more and more. A lot of young ones may doubt but if they manage to stick in due to the peer pressure and the social seclusion, they will get more and more entrenched.