The rule is 18, not 21. FFL's are not allowed to allow <18 year old persons to fondle handguns because of 18 USC 922 (x). The mere possession of the handgun by the <18 year old in that case would not meet the exceptions of 18 USC 922 (x). They cannot, obviously, transfer ownership of the handgun to a person <21 years old.
Now, there are some states that mere possession by <21 years old is prohibited by state law.
Now - FFL's MAY have <18 year old persons handling the handguns if they are employed by the FFL and the <18 year old person has written parental permission with them because that IS an exception allowed in 18 USC 922 (x).