Several years before the current storyline point, her stepfather was going to send her off to enlist in the Legion when she was 13. Currently she's likely around 17 or 18, maybe even 19. If we had a timeline of when Glenville died and how long ago that was we would know for sure how old she was, but we're kinda stuck playing guess work for how long she has apparently lived with Outsider. We just know she's lived there long enough that people just assume they are blood.