Okay, so before I get in trouble with somebody about Eight's age, I just wanted to make this blog to signify that I am possibly correct. I don't know where Eight's age came into play, as it was never mentioned in the books. I checked the companion story, "Eight's Origin," but didn't find anything—did he even remember how old he was in that story?—so there's no telling how old he really is.
I think people started thinking that Eight was 17 years old to place him around the same age as Marina. But if you remember, he said in The Rise of Nine that Reynolds met his lover, Lola, 8 years ago before that current day. He also said that by the time Reynolds told her who they were (let's say that Reynolds was with Lola for two months before telling her) Eight had increased his physical enhancements, developed telekinesis and could teleport.
Remember that the youngest age the Garde get Legacies is 11. If he developed his Legacies at 11, then 8 years later he would 19. I know what you're probably thinking—at 19 he should have long had his Major Legacy—but since the Garde are slower in development of Legacies on Earth, it may have taken much longer. For example, Number One developed her telekinesis probably at about 11-12, but then developed her first actual Legacy about 3 years later when she was 14.
It is possible that Legacies develop very slowly when they aren't around other members of their species or without being on Lorien. So Marina, who was around Ella , another Garde, for a few months, her Legacies must have grown just from being in her prescence (she developed superspeed not so shortly after Ella arrived at the convent). Five was isolated from the other Garde for a long time so his Legacies developed slower. But when the Loric Entity was awakened, he managed to use his Externa to survive being stabbed by Nine. As for Nine, I think he's just a few days, weeks, shy of getting his Major Legacy.
Anyway I'm getting off track. My point is, I think that Eight is 19 years old, and would have developed his Major Legacy within a few days, weeks. How does this theory sound, along with my crazy Legacy theory?