I would say my transition started the day I plucked up the courage to go see my Doctor and asked for a referal to see a specialist. There then followed a whole series of events and issues I had to overcome (as a consequence of my decision to transition).
I also started Electrolysis, speech therapy, buying clothes, make up, hormones, well before I actually started the RLE.
From a legal aspect in respect to getting letters for SRS / BA transition, was defined for me from the time I legally changed my name, all my documents and started living 24/7 as the requirements of the RLE. Being on hormones, living part time for a period before that had no influence on when transition legaly started for me in the eyes of the medical profession.
So, what I am saying is that transition starts for the individual, when they decide it is the thing they have to do, but there are certain criteria that the "specialists" have to see, before they will define this medically.