I tend to just say when I was a kid/teenager/college student/younger/etc...
How often does gender come up when talking about your past?
I can't think of many occasions that I really did anything gender specific, or would refer to myself in the third person. The only times I would care to tell now are the handful of times I was correctly gendered by my friends, despite presenting as a somewhat feminine guy most of my life until recently.
The way I see it, I was always transgender, and most people saw it, even if they incorrectly interpreted it as gay or bisexual male. Perhaps you were better at hiding it, and got into situations I did not, and have interesting stories you like to tell. I have interesting stories from my past I like to tell too, I don't want to pretend my past didn't happen, I was always me, I just presented slightly more masculine than now.