I think I can shed some light on your situation...

I'm 23, just finished school with a bachelors degree and all my ratings up to CFII. Looking back I'm very happy with my decision to go to school because I now have something to fall back on if aviation doesn’t add up the way I want or I loose my medical. To do it, I had to take out loans worth around $40,000 which I think is pretty reasonable considering some people I know took out 75-100K for the same result. You need to be very careful when taking out loans! Believe it or not they will want all that money back and then some eventually... With my loans I'm looking at around $400/month in payments. Consider how much you'll make as a CFI (15-25K depending on how busy you are) and the starting pay as an FO at most regionals (25K average) and it's easy to start drowning.

Whether or not you need a degree is debatable. Some will say it is not required some will say it is. I know people who have been hired recently with and without degrees so it really depends on what you want to do or who you want to fly for. My personal opinion is that a degree will open more doors for you now, and later in your career as well.

At 21 you are definitely not too old to start your career. I work with people everyday training to switch careers and most are in their mid to late 30’s and sometimes older than that. I think (don’t quote me) the average age of a regional airline new hire is 34 anyways you will be well below that even if you really take your time.

I can’t speak on ATP because I don’t know much about them. It’s my understanding that they are the “easy button” to an airline job if you have the cash to burn. For the rest of us, CFIing is the best way to get the experience you need in an economical way. You can always crop dust or banner tow or sightsee your way there but you won’t learn as much as while teaching. At first I didn’t want to be a CFI either, but I have learned so much since I started instructing and learn more every day.

Finding an instructor job will depend on where you live. I live in Chicago and had no problem finding a job even in the slow winter season.

For me, flying is the only job I can ever imagine having. I work lots of part time jobs on the side to make ends meet and it just reminds me each time how much I love working in and around airplanes.

My recommendation for you would be to finish school so you have something to fall back on and improve your resume. Try to get your ratings on the side when you can and start instructing as soon as possible. I knew lots of guys who instructed while in school even. When you get out finish up any ratings you have left and instruct until you are competitive enough to start applying where you want to fly. The amount of hours you’ll need will no doubt change by the time you’re ready so don’t worry about that for now. I think things are just starting to turn up and you are getting in at a great time.

I hope this helped, if you have any other questions I can answer just ask and good luck!