I will answer this from my perspective as a nonbinary queer freak (freak is used with all the love of myself I have. I openly use words seen as "negative" to describe myself, because I'm proud of my differences, and if you take the negative out of them, then when people use them against you, it confuses them... Hahahahaha. At least that's my philosophy.) who started the journey of figuring out my true gender identity at 27 (and sexuality too, come to think of it).
No... being a non-binary identity (genderfluid falls under the umbrella of NB) does not affect when you start your journey. Nor do any mental differences. (I say differences because I do not like the term mental illness. Illness implies you're sick. And most of the things that are categorized as a mental illness are not sick, but a different way of dealing with life, that yes CAN make us have a hard time. But they are not illnesses. It promotes a negative stigma)
AAAAAAAAANYWAY.... To say the least, a lot of figuring out our gender identity is sociological... or rather because of the world/people around us. I can honestly say, I didn't know I wasn't female until 27. Looking back on my life, I can pinpoint one instance when I was about 6 that should have been a clue... then I started feeling like my body was betraying me in high school. Taking pills to make my body work like a female's? I hated it. I remember first really starting to hate dresses/skirts in middle school maybe? But as a kid? I liked dolls as much as the next girl for example. I don't remember ever liking pink, and my favorite colors were the more stereotypical colors for boys. Did like playing with the boys though, but rarely did because as a kid, there was that whole, 'Ewww girls/boys have cooties' thing going on lol. Plus I was kinda the pariah as a kid.
In middle and high school though, people started accepting me, and as I went through puberty, I realized, my responsibility was to defy all the odds against me, or what people said were the odds. My teenage rebellion was against the stigmas attached to all I was. What it meant to be a multiracial, Mormon, adopted etc... woman with a lot of brains and a lot of anxiety thrown in. And I did. I broke all the barriers I could, all the stereotypes I could.
'Kids who grew up in foster care don't graduate high school.' I said, 'Screw you.' and graduated.
'Being adopted means no one loves you.' I ended up having a large circle of friends in both DC and Maryland.
'Being multiracial means you'll only flip burgers.' By twenty, until I was 26, I was working in a law office, and recognized for all the hard work. Despite being low on the totem pole in the law firm, the secretaries would come in and ask specifically for me or for me to watch over things being done. The companies we outsourced new me by name, etc. So screw you. Yeah, I started with fast food, but who doesn't?
'Being a woman means you *insert gender role here* ' Well screw you, I'm wearing pants, never wearing makeup, not painting my nails or going to the salon every six weeks. My parents wouldn't let me shop in the men's section... But I did break boundaries when I could.
'Oh you have anxiety? I'm sorry, we can't hire you, too many accommodations.' Well, guess what, that law firm I worked in? I have a feeling my anxiety was a contributer to why I never made Doc Center Manager, but at least everyone knew that I was going places in life. They respected me. Asked me for help. And yes, once in a while, I did need to take a day for myself, but my work didn't hold it against me.
That was my rebellion... Proving that my differences don't hold me back, but rather make me want to fight. So while society around me pressed into me the notion I had to be female because I had female sex organs, etc... and I succumbed to the belief that I was female, I did know that being a woman isn't shaped by all the sex roles society assigned to me... Just as I did with all my other differences. But I definitely didn't know as a child I wasn't female. I was too busy just trying to survive a rough world. This idea that people know in their childhood is dangerous. And every time someone says that, I just want to punch their lights out. I didn't know. I didn't show signs right away. And some of the signs might not even be real signs because they were my survival. Yes I felt more comfortable playing with guys... But was that because the girls were all cliquish and more prone to listening to their mothers putting pressure on them to accept only certain people in their groups? Or was it really a sign of my transgender nature? Or both?
Wasn't till middle school I can really say I started showing real signs of differing from the norm because I knew that's who I was. Wasn't till high school that being female started feeling flat out wrong, but I couldn't let myself be that way. Wasn't till after I miscarried my first child that I started vocalizing the words, 'I don't feel feminine,' or 'I don't feel like a woman,' or 'I hate my body' or 'I just want to rip all these out,' [while motioning with my hands to my abdomen, referring to my internal female organs.' Wasn't until after my son was born and my breasts filled out (Until then I had been at most a B cup. I'm now a DD) that I started complaining I hate my breasts. Wasn't until I was told that I have too much T in my body naturally that the world started to come together for me, but I still denied that I was anything but female. And finally, last year.... I finally searched for if other people with what I have go through what I was going through, and that's when I found out that being transgender is a broader definition than I thought, that there was an existance of genders outside the binary, that I had suffered from gender dysphoria for as long as I can remember, just didn't know it, started recognizing the signs in my past. And then I started to let go what society determined my gender had to be. And well, here I am... over a year later, and finally beginning to really be me... The point I was at when I let go of the notion I had to be female, I was literally on the verge of self-harm. The only reason I kept going was for my kids. I didn't give one what whit what happened to me, except for the fact that my kids wouldn't have me. And they wouldn't have someone to fight for them the way I do, because I know what it is like to have to fight to be seen as human like everyone else.
I still suffer that way now, but its to a much less degree... I'm happier. I'm whole. And even if my husband can't accept the 'new' me, I'm still happier than I ever was, because the last string binding me was cut loose. If someone had come up to me and told me I couldn't be trans because I didn't know in my childhood at the point I was just realizing my identity, I would have probably sunk below that point. That's why I say all this talk of transpeople know/show signs in their childhood is dangerous.