You've hit the nail on the head. A large majority.
According to the CDC, from 1999-2010, there were a total of 35,760 deaths related to firearms in the 0-19 age range. Of these, 32,398 were ages 14 through 19.
Now understand that this includes suicides, but still, that’s almost 91%. In fact, more than half, over 18,000, were either 18 or 19.
If you’re just looking at homicides, there were 23,016 for the time period in the 0-19 range. 20,845 were in the 14-19 range, again, about 91%. Again, its the same thing with 18 and 19 year olds accounting for just over half the total.
http://webappa.cdc.gov/sasweb/ncipc/mortrate10_us.html