You're still young, with plenty of time to have a career and make it to a major if you work hard. Worrying about not getting a job at a major right now is wiping before you poop. Finish your degree and your ratings, build quality time, be it instructing, small corporate flying, pipeline patrol, 135 freight doggin or however and get on with a regional then become proficient in the 121 world. Upgrade, and then worry about a major. Never let the idea stray from your mind, though. Another option is the military. They have what I would think is the world's best training, and you get to do some flying you wouldn't find anywhere else. In the military, you will be an officer first and a pilot second. There is no guarantee you will get a pilot slot, either. If you do, it may be flying helos and not fixed wing. A military pilot can certainly give you better information than I can on this.
You have valid concerns, but again, as an 18yr old, 4hr private student you have more pressing issues to worry about. I have an uncle who has been flying for a major for almost 30 years, he has been married to my Aunt the entire time, they have one of the strongest marriages I know of. That being said, this job is extremely tough on your personal life. It will take a special kind of woman to stick by your side. Personally I've been up front about my job and my career aspirations with anybody I've dated.
Pay at all levels of this job is far below what it should be. That aside, the regional pay scales are particularly atrocious. 20k for first year pay is terrible, the schedules are tough on mind and body and generally you won't be treated well. There are a lot of people who will tell you to get comfortable at a regional because moving on to a major is/will be impossible. I don't think it is impossible, but difficult, yes. Plan to be at a regional for several years before even having an opportunity to move on. Don't believe what you may hear about a "pilot shortage." There are too many variables to accurately predict where hiring will go down the road. The majors are staring at a LOT of retirements in the coming years, with age 65 finally coming into effect but rising fuel costs, outsourcing of flying to regional airlines and several other factors can all affect what happens. 9-11 was terrible for our industry, but age 65 was arguably as devastating to growth and hiring. Couple that with the recent recession and you have a perfect storm of bad news for pilots. Upcoming retirements, changes to our outdated and downright asinine rest rules (if they ever happen) and (hopefully) future growth of MAINLINE flying (i.e. NOT outsourced to regionals) gives potential to jobs being available at the majors. I should add that the more I jumpseat with majors (thank you all for the rides!), the more I hear them talk about coming for RJ flying, or they want "their" flying back. I couldn't be happier to hear this. In the short term it may harm my job potential but if it means more jobs and more flying at the mainline level, I'd be a fool to not support it and hope they get it.
As far as a 12yr FO at a regional, that is not unheard of, but is uncommon. At Mesaba? Its likely his choice. The upgrade there is not 12 years. According to this website the most junior Captain is a 2007 hire. Mesaba is currently merging and integrating its seniority list with two other carriers, Colgan and Pinnacle. Upgrade times will change but we are terribly understaffed right now. Down the road, who knows? By the time you're ready to jump into the circus that is our 121 world it may be an entirely different outlook.
As far as majoring in aviation? I wouldn't. Study something outside flying, and get your training on the side. Have a degree you can use outside of aviation should anything ever happen and you decide to/become unable to fly as a living any more.
All that being said, I love my job. I work with, for the most part, awesome crews fly an old, tired but fun to fly airplane and can't imagine doing anything else. The pay, work rules and company could be better but thats life at this point. I knew that going in. I try to focus on the good and look at what I can improve and how to do it. I had a chance to vote in a contract and help make a small difference along side my fellow pilots and hope to have more opportunities to improve the collective QoL as my career progresses.
My apologies for a rambling answer. I hope this gives you some food for thought. Focus on learning the basics, becoming a safe and proficient pilot and take things one step at a time. Looking toward your future is wise, and considering as many aspects as you can is important but ultimately you won't know if this job is for you unless you do it for a while. Listen to the others on here, take the good and the bad and weigh it all carefully. Remember that whining and complaining is a time honored pilot tradition, particularly on these boards so try not to let it get you down. Best of luck, and come back with any questions you have!