First I would check to make sure that Cabela's policy isn't more restrictive than Maine's state law, especially considering that Maine allows the private transfer of long guns to 16 year olds, and the private transfer of pistols to 18 year olds, and will even issue a CCW to an 18 year old. Maine also allows unlicensed open carry except in Portland.
This black powder age restriction sounds awfully uncharacteristic for Maine.
Maybe Cabela's is interpreting the sale of Black Powder to you the same as if a dealer
sold a handgun or handgun ammo to someone under 21 according to Federal Law which is more restrictive than Maine's state law specifying 18.
So Cabela's may just want to limit their liabilty by being more restrictive because they have an FFL and don't want to be subject to a lawsuit for any reason involving the sale of BP.
And if you do find out differently and you're traveling south to Kittery anyway, maybe you should just drive a few more miles across the Piscataqua Bridge to Portsmouth, New Hampshire.