Most people I've known who've left the Church during the years that I attended were 18 or 19 years old. I think that's the point where they can no longer hang onto their parents' testimonies.
They search it out for themselves and either commit at that point, or they leave, realizing that their parents' religion isn't necessarily their own.
I left when I was 26. From 18-25 I was so busy with mission, college, dating, being EQP, and getting my career started I never had time to question anything.
Once I was settled in my career and could come up for air, I started questioning the church. My main question after sitting through another boring sunday school lesson was...Would God's one true church really be this boring, shouldn't we be excited to be here?
You highlight the key for many of us; time to catch our breath.
The church tries hard to keep people so busy that they don't have time or attention to give to wondering about things too deeply. They focus on getting you deeply, personally invested by rushing you through the program so hard and fast that you can't stop and look at the cracks and holes.
Then sometime in our later twenties to fifties we finally have that moment to draw a deep and thoughtful breath. It's unavoidable.
All the church can hope to do is dig you so deep into the church and the culture that when you inevitably take a look around, you dare not leave for all you risk losing.
Yessss. It's boring as ditchwater. I asked a leader about this once. He said it was a duty to attend, and you will not be bored if you invite "the spirit." The only spirit that helped me get through those awful 3 hour blocks was the spirit of semi-coma.
Actually, lots of young members are leaving. I firmly believe that is why they lowered the missionary age. As soon as kids get a taste of freedom (leaving for college), for many, they're done.
But, get them into a mission right away, then get them married, and they've got a better chance at keeping them in the fold.
Neither of my daughters are believers. One (23) is married to a fabulous young man with enormous LDS privilege. The other (22) is married to an asshole who is in love with himself and his make-believe priesthood power. They have LOTS and LOTS of friends who are full-out "apostates" or disbelieve but are pretending like they are LDS for BYU, family, acceptance, etc. Few of their friends are actually believers. The missions didn't help them with their testimonies, btw.
There is a huge amount of spiritual/emotional pain in this age group. Their foundation of belief is being pulled out from beneath them, as one by one, people they looked up to leave. I feel especially bad for them because they are often newlyweds when it happens--ill-prepared for life, let alone dealing with just being married, school, babies...and finding out it's all a fraud. My daughter who has been married a few years has MANY friends already divorced, because one refused to live the lie the other chose to believe.
I was in a leadership calling recently (before leaving) and I found out about a small handful of folks in our ward who had found out the truth and left. It's not limited to a given age group, but I would assume that it's easier for younger folks to leave as they may not have as much invested.
When telling someone very close to me (19 y/o), about my apostasy, he told me that he had already found out the church history problems himself (via the internet), but was afraid to bring it up.
I suspect that word is getting out and folks are dealing with it differently. There is an effort to isolate/compartmentalize those who know the truth from the other TBMs to avoid the spread of the cancer of truth.
I think if it wasn't for the singles wards, more would be leaving earlier. The gospel isn't true enough to hold people together. There has to be a social component for everyone. If that balance is not met, our nature is to look for something else.
Lots of young people bury doubts to please parents and get social strokes. One of the un-written rules of Mormonism - Guys have gotta be RMs, girls gotta marry RM - HUGE social rewards to comply, shunned if you don't. When I was this age and decided to go on a mission I was BESIEGED with praise and love and approving glances.
For starters, calling them 'breeders' is a tad dehumanizing. Whatever they choose to believe, they're still people with their own hopes, aspirations, and desires.
Many of them are out. Even those that aren't out are in general working to change the church (ie, the Building Bridges group). For some of them, they recognize the church as an engine for change and wish to leverage it; others admire the LDS community and solidarity but wish to see a bigger tent over the LDS church. Whatever their reasons, it's not really mine to judge.
I know only a handful of people in that age group that are resolutely LDS and aware of historical issues. Most like the culture/community. I was 28?-ish when I left. And I left due to a combination of factors; one was info I stumbled across on familysearch.org about JS's polygamy, but the other was that I -wanted- to leave. If people are happy (and it's perfectly possible to be happy as a member of the church), then they'll want to stay.
It's the same reason people remain Catholic or Muslim. The Catholic church has a much more sordid history than the LDS church (they've had 10 times as long to do bad things), yet people remain Catholic. Unless they've got a reason to choose something else, they'll remain where they're most comfortable. You can think of it as cultural inertia -- people will tend to rest more comfortably where they're already at.
Please don't call them breeders. The reason is mainly due to lack of independence. I got out of BYUI at 23 and didn't have a good solid job until 25. So by them I could actually think for myself and have some space not to just worry about school and getting a job.