How does an E6 have 6 service stripes?

I don't know if the length of service limits are still the same as when I was discharged in 1990 but as far as I know an E-6 is allowed 18 years in service and an E-6(P) is allowed 24 years of service. It is not only quite normal for a SSG to have 18 years of service or more since it is common for someone at that rank and length of service to end up on a promotion list.

Some MOS's are harder to make rank in than others. It could be that the Sergeant had a break in service and came back in and didn't have enough time to make SFC before he retired. He could have been medically retired. There are many reasons that a soldier retires as a SSG. Most retire as a SFC in the Army but some do retire as a SSG.

Since you posted it. Have you really thought about your question?

How? As you said, it means he has 18+ years of service. Emphasis on SERVICE. Why would anyone assume that it's due to some reason other than his own preference; limited promotions in whatever his MOS is, and so on? I've known career E-6's in the Navy, Marines and Army who retired at that rank.
In some MOS', (in my experience) once you're past SSG, like the MP Corps, you're more or less guaranteed administrative jobs - excluding Combat MP units. In garrison, I've never seen any NCO E-7 and above work road duty. In some units, they rotated as the MP Duty Officer, and most were relegated to MP Opns or PMO Opns. I was a career MP and up for E-7 and dreaded the idea of not being on the road. However, the MP Corp had been putting emphasis on Combat MP's, and not garrison. Fate intervened in my case and I ETS'd to care for a multiple handicapped daughter. I've had more than a few MP buddies who retired as 1SG's, and SGM's and they all told me that had I stayed in, I'd have hated how the MP Corps changed. Hard to say, but pay and benefits aside, going above SSG (to me) was kind of bothersome.

A simple question. It deserves a simple answer. There are a number of ways for a SSG to accumulate 18 years of service. For one, this individual could simply have not been very interested in promotions. Thus, they occur less frequently for him. Another way could be through Article 15 action(s). There are several ways to get 15'd; some are unfair, others are just bad luck, and others are one's own stupid dumbassedness (no offense meant, we all have our stupid dumbass moments).

However it happens, I might actually recommend That Guy as a mentor. Looks like he's been around the block, like, a lot!

Many MOS situations have limited slots for promotion over E6 because they are very specialized and have only a limited number of units to promote into.

Seems to me you’d be better off asking the SSG yourself, instead of airing out your laundry here. Nut up and ask them your question like a man

there are some real shit posts here. Frankly the Army has made the empasis on rapid promotion to the detriment of the service. i see an e6 woth 18 years if service and i see an nco who probably puts others above himself daily, is truly technically and tactically profecient. from my experience the hotshots who made sgt in 3 years, and ssg in 5 generally focus on what will make them look good and generally suck at developing soldiers. just my 2 cents

He has 18 years of service.

Well the first thing you need to do is look at yourself and ask yourself, what the fuck concern is it to you PRIVATE., Next thing ask yourself what the fuck business is it of yours PRIVATE,, whenever you are in doubt refer to statement one