I made the decision to identify as trans*, rather than tenderly wish I could magically alter my biological gender but otherwise write it off, when I was twenty-one. It's been one fast-paced, all-too-slow, chaotic tumble since then.
Though if we're including when I'd thoughtlessly wish away my birth-assigned sex, that was when I was fifteen or sixteen. I had no idea what "trans*" was then, and couldn't have confidently told you the difference between a dragqueen and someone who was genderqueer at the time. (Though who can confidently categorize anyone at gunpoint, I suppose!)