You can get a Texas licence to carry a handgun before you turn 21 if you serve in an active or reserve component, instead of waiting until 21 like everyone else. You still cannot buy a handgun before 21, but you can legally possess one.
Apply for the licence to carry through the Texas Department of Public Safety handgun licensing service and submit proof of your current service to claim the age waiver.
Your military ID or current service orders, plus the standard licence to carry application materials.