US Marine ranks

Background: You'd enter boot camp as a private. If they're still offering "special enlistment terms" for qualifying candidates (ie., 6 year enlistment), you can earn a PFC (Private First Class) upon completion of basic by signing up for a 6-year enlistment. Also, the top 1-2 grads of basic can be awarded a "promotion" (also to PFC). Other than that, it's a level playing field. You're a private.

Seven years later...no combat promtions, etc. You could realistically be a Sgt or SSgt (but it really all depends on your MOS, as in military occupational specialty). Sgt is an E-5 and SSgt = E-6. You more or less earn a promotion a year up until say E-4 Cpl (the NCO ranks). Then it's a lot more competitive. I was in 4 years and got out as a Cpl. I came out of boot as a PFC. Hope that helps!

Cyia said:

Assuming someone enters the service straight after high school, and has an exemplary record, what's the highest possible rank for someone about 25 years old? Or, at least, the most believable one. (Assume usual chain of command with no special promotions due to combat situations.)

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My brother entered as E-1 (lowest enlisted) (Vietnam war era) By the time he was out after his normal enlistment ... he had been all the way to E-6 (Staff Sergeant) and back to E1 a couple of times. He was a bit of a discipline problem, but he was already in Nam, so all they cold do was demote him.

His son went in as E-1 and is now (at age 24) a W-2 as the crew chief of a helicopter squadron ... he is such a jarhead he drinks gung for breakfast.

Here's a site where you can calculate the time in Service TIS and Time in Grade TIG against the age of your character. I guessed he/she would be a staff sergeant E6 or gunnery sergeant E7.

(From RJK's link): this is the key factor I mentioned regarding MOS...I was in obscure MOS so it was really hard to make NCO.

The Marine Corps takes the number of "slots" they have for each enlisted rank, above the rank of E-3, and allocates them to the different MOS's (enlisted jobs). In other words, MOS 123 may be allowed to have 5,000 E-4s at any point in time and 2,000 E-5s, and MOS 456 may be authorized 7,000 E-4s, and 5,000 E-5s (as a general rule, the higher the rank, the fewer positions there are).

In order to promote someone (above the rank of E-3), there must be a "vacancy." For example, if an E-9 retires in a certain MOS, that means that one E-8 can be promoted to E-9, and that opens an E-8 slot, so one E-7 can be promoted to E-8, and so-forth. If 200 E-5s get out of the Marine Corps in a particular MOS, then 200 E-4s can be promoted to E-5.

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There are also officer training schemes for graduates, so they could, realistically, enter the nacy as an officer at 22/23

As Rowan mentioned MOS in the Marines and Army, Rating in the Navy, not sure what they call it in the Air Force, but they all have budgeted numbers of openings. When I was promoted to Master Chief Operations Specialist (E-9), There were 8 openings throughout the U.S. In another year there were as many as 35. It depends on the Defense budget and the number of retirements, promotions, or whatever else causes the opening to exist.
The candidates are graded, based on their annual evaluations and then for the highest enlisted ranks, a selection board evaluates and selects each position.
I believe the Army, AF, and Marine corp, prefers to keep the member in the unit, rather than allow him to transfer to another unit where a spot at a higher rank exists. The Navy will transfer the member if he asks.