How does an E6 have 6 service stripes?

He is either in a small MOS with little turnover. (Can't promote if there is no place to put you) or he came from the Reserve or Guard or he has been in trouble at some point and lost a few stripes. In any event, does it really matter?
My Uncle did 39 years in the AF Reserve and he is a retired Tech. Sgt. What does that say about him or how good an Airman he was?

Depends upon his MOS also, the smaller your CMF, the more competitive SNCO promotions tend to be. I got promoted to SFC at 17 1/2 years after switching my MOS from 31B (95B for old schoolers) to 92G (94B for old schoolers), so it is not at all unusual.

At 13 yrs in i was E5 and had been to the promotion board at 10 yrs but I dragged my feet on getting the points I needed to make E6 for 2 reasons 1 my job was what I loved the most and making E6 would have taken out working with the line companies and put me sitting in an aid station 2 I spent more time making sure the Medics working under had what they needed to get promoted . The funny thing is my orders for E6 came down 10min after I had officially signed out of the Army

I found this to be the norm in the MP Corps, for there were plenty of NCO's that retired with ten years as an E-6, back in the 70's and 80's..Kid do your research before you post, for you will offend others, Most non combat arms MOS's have a high cut off scores for E-5 thru E-6, and putting together a E-7 packet was next to impossible for most. I had 7 years in as an MP with a break in service went from 5 back to E-4, Changed my MOS to Inf went into a Drill spot, and they through the rank at me. I had 3 NCO academy's as that E-5 MP and that was it Buddy.. Hens the term Hurry up and Wait !!

This SSG is probably in a MOS that promotion points for SGT and SSG were high...I was a MP and promotion points were crazy high for SSG it was 798 out of possible 800 for a long time..that means you have to max out education both civilian and Military also max out pt score and weapon score also max out awards...thats a challenge when you factor in a few deployments. ..dont judge this SSG....simply ask him

I know a MSG in the National Guard that took a voluntary reduction in grade to SGT for an AGR slot.

No real time to research this but I used to jokingly make fun of being SGT of the Army in National Guards. If there is not a position for you to advance in your state you can spend many many years in same grade. Active totally different.

Please tell me you are joking. That is a very stupid question. What is abnormal about an E-6 with 18 years? That is average for ALL branches of the armed forces!

SSG probably been busted down several times.. Article 15s etc... That's probably why he's a SSG with 18 years of service.

What has been hinted at (and may have been mentioned in someone's post) is that each MOS is structured differently. In one particular MOS there are 30 E-7 positions. In the entire Army. So there are a lot of tremendously talented and high-speed Soldiers who retire as SSGs in that field (and walk into high-paying civilian careers.) That is just one example, there are many others with similar bottle-necks.