I wouldn't say a young age, but the Night Watch series by Sergei Lukyanenko has a similar element to that. The lead protagonist, Anton, is a member of the Night Watch while a young friend and neighbor of his, Kostya, is a vampire and therefore destined for the Day Watch. Kostya is not usually a focus in the majority of the novels, but it might be what you're looking for. He is mentioned frequently enough, and the growing idealogical conflict between the two is clear.

(I'm trying to avoid spoilers here, so I'm being intentionally vague and ambiguous.)