Recently there was a wonderful article in the Washington Post written by a millennial mainly for Protestant churches, although the thrust of the article could just as easily extend to Roman Catholicism. The title, "Want millennials back in the pews? Stop trying to make church ‘cool,'" gives the gist of the article, found here:

https://www.washingtonpost.com/opinions/jesus-doesnt-tweet/2015/04/30/fb07ef1a-ed01-11e4-8666-a1d756d0218e_story.html?utm_term=.8716a7384258

A couple of quotes: "Millennials are not disillusioned with tradition; they are frustrated with slick or shallow expressions of religion.”

As for why young people leave religion, this supports VictorVictoria's point: "I was looking for a truer Christianity, a more authentic Christianity: I didn’t like how gay, lesbian, bisexual and transgender people were being treated by my evangelical faith community. I had questions about science and faith, biblical interpretation and theology. I felt lonely in my doubts."