Since Green Lantern Corps (and the rest of the coloured corps in their own ways) are an intergalactic police force, there can't be a teenage lantern because for human age standards the youngest you could be to become a lantern (cop) is 18. Edit: Green Lantern is also referred as an intergalactic military force so I think there could be a teenage lantern (16 at the minimum) but I think the Guardians would not want a child to be in cosmic danger.