Until the appearance of The Urantia Book. We were all left wondering about all of those years between the time Jesus was found in the Temple, and the beginnings of his public ministry.
Following are three direct quotes from the book that we feel address your question. The first quote is from Jesus as he spoke to the two brothers from Emmaus, not long after his resurrection. The other two are from sections near the end of the book entitled, Meaning Of The Death On The Cross and Lessons From The Cross.
Parts I-III of The Urantia Book cover your question in detail, Papers 21 and 33 specifically; also Papers 119 and 120 just prior to the "Times of Michaels’s Bestowal" Paper will help you understand these concepts.