For the Army there aren't any health benefits until you report to BOLC. Most LTs that I know who were waiting to go on AD orders stayed on their parents plans.
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We've heard from Army and we've heard from Air Force. Surprise! They are different. Anyone know about Navy/Marines? It's enrollment time! Of course, unlike Bug'sMom I still have a year to get my answer... and even DS could get the answer in that amount of time.