I'm almost 30 and those who left did it when they were 18-23. Now those who are still in are married or very close to get married though many are not doing too much. Then, there are the MS and pioneers who are the least and the true belivers.
I think the first group represents 80% of the people my age and the second 20%. I guess there are many who don't believe the doctrines but don't want to change their lifes and get into problems.
I can understand it, it's not easy just to quit the religion and lose everything in your life. And the older you are, the harder it is to start a new life from scratch.