Hey, active duty marine here sounding off.
I came in when I was 21, two semesters away from a degree at a very liberal liberal-arts school. For the naysayers, anyone can adjust. Yes, basic is a mindf***. But, as someone said above, it's more a mindf*** than it is a physical challenge. Yes, it's pretty much always uncomfortable, you never really have time to "rest" other than in the rack at night, but most nights you will get your full 8 hours (9pm to 5pm, taps to reville) unless you're firewatch. If you're physically fit coming in, it really wont be much of a challenge. I actually lost fitness in basic.
What basic will give you is a desire to kill everyone. No joke. From "haji bob" to your drill instructors to your dumbass f***up rackmate who can't get his boots on fast enough, you will want to kill everyone, or at least severely hurt them. The biggest thing to remember about basic is that everybody pays. One person f***s up a drill movement, the whole platoon of 60+ might be spending time in the sandpit (and, btw, the vast majority of your time will actually be consumed by drill. If you can find a marine to show you how to drill before you go, you'll probably make squad leader real quick. Being photogenic doesn't hurt either) The second thing is that for three months, your life is not your life. It's something that you can't understand until you experience it, but every waking (and sleeping hour) belongs to your own personal cross between the devil and jesus, the United States Marine Corps Drill Instructor. It's the weirdest thing, but you will hate these men more than any other person you have ever met, and in the same instant respect them more than your own father. Strange brew.
Yes, enlisted basic is a brainwashing machine. You will do martial arts training in pits of black mulched rubber in 80 degree heat and full cammies while screaming "Kill! Kill! Kill!" each time you hit the bag. When you're delirious from the heat and you've got a drill instructor (or three!) screaming at you while beating the shit of the pad, I assume that does something to you. But, after about six months to a year, assuming you are an individual of generally sound mind and solid caliber, you'll get over the murder instinct. The other thing you'll get used to is the instant - INSTANT - obedience to orders. That's what a lot of the older recruits have problems with. If you're 25, there's a good chance you will have a drill instructor younger than you. But you have to listen to them. Because if you're the one to f*** up, everyone will pay for you. I mean, if you think about it, that's what the marine corps needs from the enlisted marines. Cannon fodder, go there, kill that. Go there, deliver this. Go there, push that button (if you're in a support MOS). It's this way because most 17, 18, 19 year old kids straight out of high school need this discipline to do the right thing at the right time. Trust me, Drill Instructors do not give a shit if you have a doctorate or can run a 2:20 marathon (and if they do, they certainly won't let you know about it). SO, that's an abridged explanation of what you'll encounter in enlisted basic.
If you want to retain a bit more autonomy, I highly recommend the officer corps. It's a bit more of a risk, because, unless you legitimately like snap your leg in half or something similar, if you flunk out of OCS, that's it, you're gone. OCS is much more geared towards evaluating the individual than evaluating the group, as, opposed to the enlisted Marines function of providing mass to accomplish the mission, the Marine Officer must organize and supervise the movements of the enlisted to ensure mission accomplishment. In any branch of the service, the officer side is the thinking side of the house. That's why you have to be a college graduate. Plus, your base pay is better, and the eyes on you when you aren't at work are much less demanding. What really ends up getting the enlisted who join when they're older is the micromanagement that happens with the junior marines. Even if you're a shit-hot motivator, you will be living in the barracks (unless you're married), and will have to deal with the fact that you have a room shittier than your first college dorm room, the chow hall sucks but they're going to dock it from your pay anyways, and at any point in time, at the discretion of your chain of command, you may be called to stand subject to a room inspection, and, depending on the mood of your immediate superiors, or maybe depending on the mood of your superiors twice removed (because, remember, in the Marine Corps, shit rolls downhill..!) you may be called upon to field day your room, which means, that, as a 25 year old man, you will be punished for not keeping your room to a standard that even your mother never held you to (and I give you the benefit of the doubt that you may have had a Mennonite upbringing), and be required to clean it for as many hours as it takes for your chain of command to cool off at whatever they're really pissed about... Really, what I'm trying to say, is just go officer. The enlisted will probably talk shit about you behind your back, call you a boot LT who's been in for less time than they have, but as a Lance Corporal (about to pick up Corporal, knock on wood) with buddies who went officer right around the same time I came in, officers are the shit, and they might have a lot on their plate, but trust me, it's not as meaningless as the shit we've got on ours. Sooo yeah, go officer.
Went through basic in the summer of '08, btw, so I'm not too far removed from it. And I could probably fill you in on OCS/TBS too, so fire away if you've got questions.
And I made it sound like it sucks, but you will never know the same sense of pride as having earned the title of US Marine. Short story: At 17 I walked on to my high school XC team ran my first 5k in 28:36. Went to a small NAIA college, worked my ass off, got up into 100 mile weeks, ran 16:08 at age 20. It never came easy and I had to fight hard for what came easy as hell to other people, and I was (am) pretty proud of that. But when you get that eagle, globe, and anchor in your hand, you become part of something that I still can't find the words for. (And I'm secretly a peace loving hippie, so, if I can feel so strongly about it..) If you want a challenge, and you want the pride, don't even look at the other services (unless, unless, you look at being a navy corpsmen. Those guys are f***ing awesome. Bravo Zulu for all you squids (whatever the f*** that means)). The Navy is chill, but they're just not the marines. After that, the Air Force is affectionately known as the chair force, and the army, well, the army is just nasty... Same fight, alright, I'll give 'em that, but if you're really thinking about it, go down to the Army recruiting station, walk in, look at all the smoking and joking, listen to all the bonuses they want to throw your way, watch all the pandering, and then go a few doors down to the Marine Recruiting Office, and notice how it's almost dead silent when you walk in. Watch as they sit you down and ask you what you think you can bring to the corps. Listen as they tell you that they won't give you the same bonuses, but they'll give you a title that's worth twice as much. Try it. You'll see. It's weird as hell.