Kevin Drum reproduces a chart from the Wall Street Journal that shows the results of a survey in which the percentage of people in the US who identify as Christians decreases as you go to younger age cohorts, while the number who are unaffiliated increases.
He concludes that Christianity is dying out in 21st century America as it finally catches up with Europe that saw a major drop with identification with Christianity in the last century. But such claims can be challenged by the assertion that as the young people of today get older, they will become more religious, so that there will be no long term changes.
So what we need is to compare this chart with a similar chart made decades ago to see if there is an actual drop across all age groups. Maybe the Wall Street Journal article has this information but it is behind a paywall and I cannot access it.