An enlistee aged 18 or older was given the option of enlisting in the Regular Navy, which meant serving on active duty for a minimum of six years. Few chose this, although my older brother did, having been told (inaccurately) that Regular Navy people were favored as regards promotion.
To be sure, there were people in the Naval Reserve who were called to active duty when war seemed imminent, among them a SM2 I served with in the Armed Guard. Some of these men had gone into the Fleet Reserve after a hitch with, or upon retirement from, the Regular Navy.About officers, I could offer some thoughts but they would consist mostly of speculation.
This is far from complete, and may not be altogether correct, but I hope it's helpful.