The difference between these two sentences is more connotative. The first one transports an expectation from the past that finally got concreted and an expectation that the action still goes on presently or until a defined moment. The idea is that you "passed" through a transition in your life, from one stage to another.

yes, the two words are correctly, but is more usual say "comecei".

Yes, they mean the same thing. I think sometimes they are interchangeable, but there are cases in which there would be a subtle difference, but I can't figure out now how to explain that difference :(

Yes, exactly