Per NRA Range Fees webpage, the price for "Shooters under the age of 18 sharing a lane with an adult - free." That suggests 18 is an adult to use the range. However, I have nothing to affirm or contradict that. The NRA Range webpages say to contact email@example.com for questions or more information.
Unfortunately, state law and/or federal law won't help you. The NRA range is private property. They can establish whatever usage rules they want so long as they don't violate civil rights (race, creed, color, sex, etc). Unfortunately, 18 to 21yo's are a discriminated and disenfranchised group. They can vote and be sent to war but are prohibited from other adult franchises.