The case for: There’s certainly a correlation between the decline in crime and the increase in the median age of Americans. The crime wave hit when baby boomers were in their teens, 20s, and 30s — the ages when people are generally more likely to commit crimes.
The case against: The implication of this theory is that crime rates among people in a particular age group didn’t change that much — that there was less difference between the rate at which 20-year-olds committed crimes in 1990 and 2010 than there was in the number of 20-year-olds at that point. But that’s not really what the evidence shows. During the crime wave, crime rates for each particular age group rose; after the early 1990s, crime rates for each particular age group fell.
The bottom line: A small effect. A 2015 analysis by the Brennan Center suggests that the number of 20- to 30-year-olds in the population does have a small impact on crime (though not the number of teens). But we’re talking about a difference of a few percentage points in the crime drop during the 1990s — and nothing during the 2000s, when there weren’t big demographic changes in age.