Skye Jethani, pastor and senior editor at Leadership Journal, writes about the exodus of Young Adults from the church and proposes an important reason: The church has very little to say about what's core to a single 28-year-old.
With more young adults delaying marriage longer, and with most churches implicitly or explicitly designed to serve families, there is little reason for a single 28-year-old to engage. Realizing they cannot rely upon family felt-needs, but still wanting to reach young adults, some churches reach for the only other tool in their box: mission. ... But missional ministries that try to engage a single 30-year-old don’t accomplish this because they ignore what’s at the center of his life to nibble at the margins. And what is at the center for most young adults? Vocation.
You may also want to read Dick Halverson's thoughts.