Well, I figure I might as well add in my two cents here.
Just so I can make clear where I'm coming from I'm a 'neo-convert'. I am one of those people who was raised Catholic, left the church, and now I'm trying to struggle my way back. In the time I've been re-learning the Catholic faith a few things stood out to me.
First, there isn't much of a support network for unmarried young adults. Churches in larger cities tend to have these, but you get away from those areas, and that age group is pretty much expected to find their own way in the church. While I doubt that the group of people that are non-practicing Catholics is exlucive to the 'young adult' age range, I am certain a good percentage is.
Second, one of the biggest challenges of Catholocism in my experience is that it is a moderate religion. We must mind the virtues and try to avoid both the defect and the excess of them. In a society that pushes to show no restraint either direction, attempting to be a moderate person is extremely difficult, and without a proper support network, extremely lonely.
Compound these two factors with a general prejudice against Catholics that can be prevalent in any area not predominantly Catholic, and many will try to hide any overt signs that they are Catholic. The main issue that plagues my generation, from my perspective, is culture shock. If we were not raised in an overtly Catholic home, taught in a Catholic school, or really socialized in a Catholic setting, we can find it difficult to balance our Catholic heritage with the society around us.