That's not true of all rental companies. Each has different stipulations and it can vary by state as well. For instance, state law allows anyone 18 and older to rent a car in MI and NY. Outside of those 2 states, most rental companies will rent to anyone 20 or 21 and older. They generally charge an additional "Young Driver fee" but some companies waive it if you're a member of certain organizations like AAA. Government and military personnel can also rent vehicles at 18 and are generally not charged the young driver fee.
As a Marine and later as a vet, I've rented cars and moving trucks when I was under 25 and never been assessed the fee or been given difficulty.