History proves you wrong :
Jeanne D’Arc - died 19
Blaise Pascal - mathematician at 11
William Sidis - went to Harvard at 11
Pablo Picasso - painted at 8
Bobby Fischer - chess Grand Master at 15
Mozart - died at 35, started composing as a child
There are long lists of child prodigies.
British Army you can join at 16
US Military you can join at 17
Legions and legions lied about their age to join the military in the various wars in the last 150 years.
Calvin Graham became the youngest decorated hero during WWII at the age of 12 as a Navy gunner.
Whats your problem with teenagers?