I started feeling out of place when I was about 5ish but I was 10 when I realized I was probably a "transexual".
About 17, close to 18. It took a few more years after that for me to actually come out and start transitioning.
I always felt like my body was wrong but I didn't work out that I was trans until 2015, about halfway through the year.
When I look back on it I knew when I was about 10 but I tried the whole fake it until you make it thing(didnt work in the slightest) and it took me quite a while(about 10 more years) to truly come out to myself and accept it.
I started to privately identify as 'more girl than boy', i.e. effectively transfeminine (though I didn't know of any such labels or that transgenderism was even a thing), when I was about 5. Then I suppressed it becuase of various reasons including bullying, trauma and introvertedness, until I was about 19, when I went through a number of non-binary identitites, gravitating toward femininity, until settling on female.
I haven't figured out I was some kind of trans before I was like 23. Looking back I see plenty of clues (including the fact I was only attracted to women if I imagined being also a woman), but because I have a lot of typically male interests I guess I confused it all with just sexuality or ignored the signs. So I'd say vague clues at puberty that I can only identify in hindsight, and actual questioning slowly starting past 20 and peaking at 23 till now when I'm fairly confident in my feelings.
I'm not what you mean by "figure out". I'm still uncertain. But I figured I have a "boy brain" at 14. I didn't think it means I'm trans though...
I knew I was "different" my whole life but I did not know my true gender identity until 2 years ago when I was 15 and found the term "Genderfluid." Im 17 now and happier than ever. (!)(!)(!)(!)(!)
It was definitely a journey through identities, I identified in a lot of different ways from under the non-binary umbrella but it looks like I'm a plain and simple slightly feminine/androgynous dude.
I've hated and felt uncomfortable in my body from about 7 but never realised it was dysphoria. Started properly questioning and realising that I might be trans when I was about 15/16. Took me 2 years to come to terms and I accepted my identity as a transguy about 6 months ago.
I finally figured it out at 15, nearly 16 years old. I would have been able to know earlier if I knew being trans was a thing. Unfortunately, we learn about gay people, but I didn't hear any mention of trans people before I turned 15, and that was because I looked for it on the internet. From then on, figuring it out was easy. Getting help for transition was the real pain, though.