How can someone retire as an E5 or E6?

Unless it's changed, a SGT(P) can stay in 20 years. Seen it happen once before the GWOT started. There was a reason he was only a SGT(P) and I'm glad he wasn't put in my PLT while I was a PSG. Also, SSGs have always been able to do at least 20. If I remember correctly, during the worst parts of GWOT they're RCP was up to 24 yrs.

Before GWOT, SSG retirees were pretty common. Before I retired in '13, it seemed like SFC turned into the new SSG for retirement. It seemed that it wasn't as 'special' to be selected into the Senior NCO Corps. Many SNCOs had the same feelings.

Seems to me a real LTC would know about folks being medically retired. They would also know how long an E-6 could remain in the service.

I retired as an E6 in 1988. The Army had a program for E5s with 10 years of service can retire. It was a limited program and the E5 was advised to take it. And he did. That program existed for about a year.

I am not retired. But I have over 20 years of service. I had several breaks in service. My 20 years extends from 1973 through 2011. My last enlistment in 2005 didn't have an E-6 slot for me so I took an E-5 slot in the CAANG. I was injured in 2007 and put in inactive status. So that is why I never made it beyond E-6.

They could be Guard, where there aren’t TiG limits on staying in.

Happens all the time in the National Guard. Especially in low density MOSs. if there's 1 E-8 slot in the state, 4 x E-7, 6 x E-6, 10 x E-5; You can expect people who don't want to reclassify to stay LONG stretches at E-5 or E-6 ... Think public affairs, band, plumber.

MSgt Kurt S. responding to your question

MSgt Kurt S. - I have a blue ID card, Orders, and DD214 that states "Retired". I can no longer post on that specific thread.

well u made PO1/E6 in five years in the Navy Seabees where E7 and above is tight. For advancement we in the Navy take an exam E1-E6 the tests are given twice a year E7 is given once a year in January then in May you spend in a package with other achievements that aren't covered in the exam. then in June your record goes to a review board and from there they pick who gets advance. then it depends on how many openings there are for your rate, so for my last 8 years on average they would promote from 4 to 8 E7's. and in the Seabees which at that time there were only 9,000 on active duty so it made it kind of tight. Don't know how it is in the AF for advancement but in the Navy it just makes a little bit harder but this way they get the best to be able to be called CHIEF. like my Dad

Well Sir,
I won't make any excuses, but I come from a very competitive career field where we don't have a lot E-8s and E-9s. Competitive in a sense when you really had/have to have your stuff tight...school, leadership positions, duty assignments, etc...pretty much no discrepancies. I don't know how many Sergeant Majors are there in my former career field now, probably 4, maybe 5. I know a lot of service members and friends that retired as SSGs that had their stuff tight, new their job inside out, did everything they could do to get promoted, and never got picked up to a higher rank. Some of those individuals got offered jobs outside the military because they owned security clearances, like myself.
To make E-7 or SFC in my career field is almost like making Sergeant Major because it is that small. With that said, if you get in serious trouble like me, and I'm not taking about small stuff, I'm talking about to the point of receiving letter of reprimand from a three-star General, yeah Sir, you let me know how the rest of your career is going to go after that in your records. Another thing is/was the promotion systems which was almost impossible to make E-5 or E-6 because they were up on 798 out of 800 most of the year. I mean, I can go on and on...individuals like going back to the promotion boards to raise their points because they couldn't make it year after year. I think the lowest I've ever saw the points going low was to 785...I know because that's how many points I had when I got picked up for SSG.
Last but not least, yes I could've stayed a couple more years, but I chose not to. My family had enough of me been gone overseas. As much I like the Army and the job, I just had enough, my mind and body had enough.
People get out or retire without reaching the personal goals they set for themselves for different reasons Sir. Just remember that when your time comes, with due all respect.