Should they HAVE BB guns? YES, as soon as they are able to safely carry and operate them hands off.
Should they be allowed to PLAY with BB guns? Hell no. BB guns ARE NOT TOYS, contrary to The Christmas Story.
Sometime between 8 and 16 (ymmv) you’ll know if they are capable of enough responsibility to shoot under partial or minimal supervision, but until then no.
Parent's hands off that is for you jokers out there (heh)