Just had this experience.

I'm in my mid-50s too and recently changed jobs. I had to update my resume for the first time in a long time.

I decided to leave the dates of my undergraduate and law degrees off the resume. I also left off the timelines of the various law firms at which I worked over the years. I did put the date of my, at that time, current employment to "the present" so that they would know how long I had been in my existing position. Although I went back more than 15 years, I did not have too many stops after law school so the list was not too lengthy.

Another thing to consider is the length of your resume. When I am hiring somebody, I usually like to see everything synopsized on one page. At this stage of your career, this doesn't apply to you. When you get to a certain age, you have a decent number of accomplishments, relevant awards and other things worth mentioning so my resume ran two full pages. In addition, when I changed gigs about 15 years ago, I had put together different versions of my resume that highlighted the relevant experience for the particular job for which I was applying.

You definitely should mention your time in the Navy. That's always a plus factor when I'm hiring and I know a lot of employers look favorably upon that. I agree that the dates are not important. It doesn't hurt to mention your rank at retirement (assuming you got past midshipmen).

Good luck.