You can apply for PLC while a freshman in college if you want too. The OSO will prepare you for the training throughout college and you'll go those two different summers for 6 week each. You will compete for a Pilot slot. They are extremely competitive as they are in any service. IF you maintain your physical qualifications, grades and graduate training you'll be commissioned a 2ndLt after graduation. You will then go to The Basic School (TBS) where you will learn about every MOS (job) in the USMC and become a basic infantry platoon commander. This school is where everyone without an aviation guarantee competes for their MOS.
Then some of the real work begins for you if you have made it this far.
Everything you do from this point forward has relevance to your possible future. Every test, every physical fitness event, every inspection, every role you pay, etc.... figures into the path afforded you throughout flight school. the better you do the better chance that you'll have that YOUR choices have an influence - and that still isn't always true. The needs of the Corps come first.
What you have heard about the selection rate is true except the fewest go Multi-engine (KC-130s) and there are the Tilt-Rotor guys now too (MV-22s). Any path can lead you to the ariline after your initial commitment is up - it just depends on how you play it.
You'll hear this from plenty of other guys - chose the military route first and foremost because you want to serve your country. If you don't - you will find that your time in military service will most likely not be what you thought and the Marines will certainly draw it out of you. That route is the most cumbersome to get to those eventual wings - but we do it because we wanted to be Marines first. Once you've decided that route is for you - I doubt that you'll ever be disappointed no matter what you fly. The friends for life that you will make and the experiences that you will have will be second to none. You can take those experiences and that training into any aspects of life after the Corps - flying or not.
As for other aspects of the airline life - I can't help you there - but Google 'The Truth About the Profession' and read it a few times over for a good and honest overview. Then spend time reading this forum. Most questions you can think of have been asked and answered many times over. For more info about the USN/USMC/USCG sideof aviation, visit airwarriors.com; BUT READ BEfORE ASKING.