OK, they weren't exactly natives, but they weren't ethnic English either (albeit they had some English ancestry in Pitcairn). English ancestry from back when the age of consent was 10-12 in that society, so having an unofficial AoC of 12 was keeping in line with British thinking at the time their British ancestors made contact with their Polynesian ancestors. It doesn't appear that St. Helenans have much, if any, British ancestry.