Dear IceRider -- The answer to your query is conained, if only implicitly, in the "Ranger" paper, where it tells you that:
(1) The vertebrae are the last part of the skeleton to complete growth;
(2) The neck vertebrae are the last of the vertebrae to complete growth;
(3) The basal neck vertebrae are the last of all.
No matter what age the horse, the neck vertebrae are always aligned in an "S" shape as seen from the side. However, the proportions of the "S" change, so that foals generally have more deer-like, upright necks with a deeper lower curve and less of a turnover than they will when they have completed growth.