Older adolescents, ages 15 to 19, accounted for 82.6% of gun-related deaths in 2021.
Poking around the CDC website adolescence is defined in multiple ways but generally includes ages 12-19, so might be better described as "teens" even though 18+ is a legal adult. I think it's being treated here as more of a developmental stage than a legal one.
Digging into it by age, from 2018-2021 firearms made up 2,149 out of 22,545 total deaths (~9%) for the age range 5-14 in the US. Looking at 15-19 this increases significantly to 13,321 out of 46,323 total deaths (~29%). This corresponds to increases in both homicide and suicide by firearm for older adolescents.
Quoting this just to make the point that firearms do have differing impacts on younger and older children, and that extends to race and income level as well. But whether guns are the leading cause of death for an age group or not, the end result is the same: more dead kids.