Why older mass shooters like the California gunmen are so rare

The gunmen in both of the recent shootings in Monterey Park and Half Moon Bay, California, had an unusual profile compared to most perpetrators of violent crime: They were both senior citizens.

The Monterey Park gunman, who killed 11 and injured nine before fatally shooting himself, was 72. The Half Moon Bay gunman, who killed seven people before he was arrested in what police have characterized as an act of workplace violence, is 66.

Mass shooters of that age are rare, especially those with no prior criminal record, as was...