Where do Americans draw lines when it comes to guns and teens?

In the United States, a male under 25 is now the most likely suspect of a mass shooting.
Consider three examples from this year:
A 22-year-old wearing body armor and wielding an AR-15 attacked an LGBTQ nightclub in Colorado Springs, Colorado, in November, killing five and injuring 17.
A 19-year-old New York man pleaded guilty Monday to domestic terrorism motivated by hate, which his lawyer said “represents a condemnation of the racist ideology that fueled his horrific actions on May 14.” He will be sentenced to life in prison without...