There's interesting psychology at play here to be sure - I was roleplaying before I was 18 and getting into 18+ circles, but that was a matter of mindset and different circumstance, and not something I could turn into a usable guide for others.

Instead I'll speak for myself now, as I developed past the mindset of those early years and have put myself firmly into the 18+ search camp.

I'll share the main reasons I'd look by or likely decline if you approached me for roleplay.
 - 18+ is a matter of liability in many cases. I get into legally 18+ content frequently. If it's not for sexual content, it's because I straight up explore very adult material. The last thing I need is a parent on my ass. If I consciously know you are not 18, I am liable because of it due to where I live. Regardless of what we're doing I am now walking on a certain set of legal eggshells. I don't need that in my life. Roleplay with me is under the automatic assumption you are the proper age. If I don't know and you strike me as mature, whatever. If I do know, that changes the dynamic regardless of how mature you seem. I can fight with ignorance, that goes away once you make a front line opening of the fact you're 15.

I had to cut off someone who was 17, both physically and in maturity that I knew might have been on the border. It sucked, but due to the content matter and my stringent policy on how to not get in shit, it had to happen. The player is now 18+ and I welcome contact anytime.

 - 18+ is a generic, but often accurate form of vetting. Legally or not, it's simple fact that on the average, people who are 18+ are more mature or at least functional with the subject matter I explore on average than not. Now again, I don't have any qualms with the idea of having roleplayed with someone below 18 who never brought it up and who was mature enough. Why? I did that myself. I didn't go out with age. I was a slippery little shit who just made it about the roleplay and did all sorts of other things to pass the time (aside from life itself) and it worked. If someone else does that to me, good for them, they've earned it and they're certainly smart enough not to bust it by opening up out of the blue and going 'oh and I'm 16' because it's almost certain they too are aware of a level of liability even if they aren't in material that would result in legal issue. I know people under 18 can have the maturity. Usually they don't. Usually those people oust themselves. Sometimes they're above the legal age and make decisions like they're 12. Don't worry, I acknowledge that too.

 - Thou protests too much. This is the biggest thing that would turn me off from roleplaying with you.

It's not because of your age itself. It's because of your focus on your age. Your total lack of awareness of the liability involved, or the fact people begin making assumptions when they know about things. Instead of focusing on the roleplay and maturity you bring, you prominently focus on your age, right down to your signature. You are defensive about it. It is the stereotypical "but I'm 12 [b]and a half[/b]," or "hey I'm a kid but you don't know me" while seeming to miss what people are actually saying.

Aside from that you have a tendency towards snap statements. It's harder to explain, but there are signs from the writing style itself.From that I assess, right or wrong based on what you show me, that you would not be able to get me on maturity. You wouldn't be able to hide it if all you did was not bring up the age, per the interest check (which is subverted by this thread, something a serious partner would likely see before jumping into a message). You [i]look[/i] and [i]act[/i] 15 as it is interpreted online. That's down two counts.

18+ also tends to offer advantages in life experience, situation handling and (per above) handling of sensitive material (though these days that becomes very murky...) all of which people would be right, by broad stereotypes that developed for a reason, to wonder about with someone younger than that. It's why you have people who tend to look for others in their age group or people who actually go out and write 25+ at times. It's a bit simplistic and I don't do it, but there's a reason for it when it shows up as something people stick to. 18+ on this site though strikes me as being used for liability and as basic vetting much more often, slightly informed by the above.

 - Other things.

Aommokkx brought up quite a bit here. For the recent history alone I wouldn't know what to make of it aside from wondering if you've actually given enough of a chance to say people are, in fact, ignoring you. It's a simple fact that most people are going to have a heck of a time both finding and keeping roleplays because this is an issue across roleplaying itself.

This is blunt and not well tailored, but it's my honest thought. If it isn't clear now, it may be more understandable down the line.