Lloyd: Why do you want to learn how to operator a tractor, is it to satisfy a curiosity, or to build and develop skills and experience for a job/business that you intend to do later in life? If you are leaning toward the job aspect of it, I have a little advice.... You want to demonstrate something that the farmer will gain in exchange for teaching you. Even though he is getting a driver in return, you also have to understand that for him, it means more supervision, more training, and more potential for accidents, if it is not taken seriously. If you are looking at a long-term prospect of of dealing with large tractors, it's best to establish a long lasting and good relationship with your mentors. Let them know of your plans and prospects. If they know that, they may be more inclined to move your progress forward as needed, instead of just letting you putt-putt around the back forty for a couple of hours.
Driving and operating big tractors is usually a tiered learning system. Demonstrate that you are responsible and careful with your starter tractor; it may be something old, faded paint, just plain awful. But after a few days/weeks of operating it as instructed, your mentor may be more inclined to progress you into bigger and more powerful equipment. Start small, learning the basics of gears and tractor operation and continue to work at refining those skills, like smooth clutch engagement/disengagement. he hardest part of driving different brands and size tractors is finding out where the manufacturer put the controls you need at. You might be a bit confused if you start out learning on something other than a Powershift or Hydrostatic transmission. Don't worry, and learn whatever you can. You'll be proud later that you can operate a Quad-Range.
Are you willing to get your hands dirty? Just driving one is just the easy portion of operating equipment. You may want to expand your horizons to encompass tractor servicing and care of them. I'm not saying you have to replace a water pump, but be aware of little things that motor/trans oil levels, operating temperatures, and oil pressures. Be prepared for something to break and how to deal with it (in other words, if oil pressure drops to zero on the 4630, don't run it until it quits, shut it down, now!).
I grew up on a farm and one of the tough things was finding good help that will take care of your stuff. I had to take care of our stuff because our livelihood was at stake, but anybody else wouldn't. That's why they always stuck me in the 9600 combine and articulated tractors. But, if you can show the farmer that you are willing to learn and are trustworthy enough to allow you free reign with his/her $200K rig, then there is no doubt that he will let you learn all that you want to soak up. Heck, he would even pay you for it if you do good enough.