I knew like... a solid 12 years before transitioning, unfortunately. One of my great regrets was to not transition sooner. Almost all of my friends have been femme and I experienced this huge hole in my heart every time I would have camaraderie with women, and it was just tough to analyze at the time and it was just the audhd rearing its head, when every single time what I was feeling was legitimate longing, extreme envy, to just be them. There wasn't so much a key tipping point as much as there was just recognizing that more and more over time and putting words to it, and when I started hanging out with other trans women, it hurt again because that never felt like it was "for me", y'know? "It's for trans women, it was for people born with that feeling", etc. etc. (Which is why I think egg prime directive discourse is complete bullshit and would have saved me years of hell, but that's neither here nor there.) After a while, I just compiled enough knowledge of my feelings, and after being an emotionless robot my whole damn life, eventually I just snapped and said "fuck it." I asked if my partners at the time would use femme pronouns for me, and the FIRST time they did, I broke down crying. I was on HRT about 3 weeks later and it was the best decision I've ever made in my entire life.

Also, if this is more than just a curious question anon, let me just say this: I transitioned in my early 30s. There is ALWAYS still time. You can just be a different gender, and all that shit that tells you that it's only for other people? It's all BS.