1. Most likely, I would think. Someone with plant or earth magic could probably use rocky soil to grow crops well
2. Yes. Magic is just like any other skill or ability, but it has to fit within certain parameters. Example, even a very strong person wouldn't be able to lift a semi, but with others' help they would
3. I would say it depends where the start is and how they discover they have it. If a three year old discovers it the same time as a 13 year old, the 13 year old would have a greater skill at refining it. But if it's found younger and trained, the skill of the three year old and thus their potential would improve.