Richard Aiken MD PhD @rcaiken
Cause of death by firearms is becoming more likely at younger ages in the United States.
The following data on the homicides by firearms [1] come from the Global Burden of Disease assembled by the Institute for Health Metrics and Evaluation at the University of Washington.
Males in the United States.
In the interval 1980 – 1984, the most likely age for death by firearms was the mid to late twenties.  Since that time, the rates have shift toward younger ages.  The following graph indicates homicide death by firearms in the more recent interval 2010 – 2016.
More currently, then, the most likely age for death by firearm is about 15 years old, roughly 10 years younger than that 30 years ago.
References
[1] Institute for Health Metrics and Evaluation (IHME). Causes of Death (COD) Data Visualization. Seattle, WA: IHME, University of Washington, 2017. Available from http://vizhub.healthdata.org/cod. (Accessed February 18, 2018)