How does an E6 have 6 service stripes?

PFC, you are out of pocket. Shut it down now. Your question is rude and disrespectful. Why or how he is an E-6 with 18 years means JACK. He is a non-com and as such has earned the respect due his rank.

OK in the Army you call them service stripes 1=3 yrs. In the Marine Corps we call them hash marks 1=4 yrs. I never knew there was a 1 yr difference between the two branches of service!

Explain for us (non-Army) types; i.e. Swabi's, Jarheads, Coasties, and civilians, are you referring to time in service stripes, enlisted grade stripes, or something else.

Six service stripes for Marines, Navy and Coast Guard is a lot for an E6 because theirs are every 4 years. Army is every 3 years, and you have to account for a lot of reservists who get stuck in a rank for years.

PFC is definitely on to something. What is RCP for E6 nowadays?

You can retire as an E-6. Nothing wrong with that.

It makes perfect sense, I retired from active duty with 20 years and 12 days service which means for the last 2 years of my active duty I was required to have 6 services stripes reflecting 18 years of service but less than 21

Back in the 1980 and pre-war in general, getting out at E6 was quite routine for some folks. Just do your job, hit the target as best you can, show up and do what's asked of you. Not hard to do back in the day...... Becoming an E7 and a Platoon SGT was not always in the cards for some folks, sometimes they just were not presented with the option or in may cases, a personal decision that they just didn't want to deal with it.

He could be in the National Guard.

Happens quite frequently in the Navy and I assume other branches are the same, depends upon your Rating/MOS. Some jobs it's damn near impossible to advance.