Boars definitely grow faster and better than gilts (I hate it when people say "boys and girls", by the way).
Butchers are used to commercial pigs, which mature a lot faster than traditional breeds, so they are used to detecting boar taint in sexually mature male pigs when the pigs are "modern" (i.e. genetically improved) breeds. That's the reason.
Personally, I've never had a problem with traditional breed boars reared to about eight months old (and friends have kept them longer without complaints).
Your choice.