I'm looking for a certain scenario:
A person for some reason looks like child as is believed to be one by everyone, but that person is in fact older (or mentally mature). Looking like a child and\or being mistaken for one often wouldn't be enough though.

Examples:
Gun x Clover - a child like looking mercenary working undercover in highschool (he is definitely not a teenager)
Ichinensei ni Nacchattara - highschooler is turned into a loli and made to go to grade school. It's probably the closest thing to what I'm looking for. In one scene he looks at his classmates and thinks how childish they are, yet in another he gets pulled in and gets along with whatever they doing anyway. He even develops some protective instincts towards the less annoying kids.
Please Save My Earth - Rin, grade schooler who is aware of his previous life (and has partially the mind of the person whom he was in the previous life)

The key elements (the more in one manga the better):

• everyone believes the character to be the kid (some may know who he/she really is, but most stay in the dark about it)
• character gets treated like the kid
• character is place among his supposed peers
• character has air of maturity and has greater knowledge and skills than the real kids
• character distances himself/herself from childish behavior...
• ...and yet sometimes actually acts like a child anyway

The following are welcome:

• as always, some romance (more platonic one is fine too) is most welcome
• character developing protective instincts towards the actual kids (extra points if he does so towards people supposedly older than him\her)
• character development...

Character can be female or male (it doesn't really matter). The supposed age can be up to late teens (if the real "kids" are high-schoolers it's fine too).

Please NO: Hentai/Shounen-ai/Shoujo-ai

Shibariya Komachi mostly

Is this me in a nutshell?
Pretty much.
[img]http://www.dragonflycave.com/wpay/ninetales.gif[/img]

Meitantei Conan
Bloomed in Action
Mahou Sensei Negima!
Mahou Tsukai Kurohime

These are all good ones that fit what you're lookin for.

The mc's in these have the condition you're after:

Detective Conan
Nanatsu no Taizai
Matentai Loki
Kazan
Sengoku Youko-( i think, haven't read in a while...)

Also it happens with one character in Deadman wonderland, though thats more of a teenager than a child and its not a main character.

The Friendly Winter fits perfectly. It's a webtoon though.

Seconding Detective Conan and Matantei Loki they fit the criteria very well. About Deadman Wonderland:

The one meant is the Antagonist and he switches between bodies, therefore having an Body that doesn't fit his actual age.

SHOUJO:

Dear Mine
MAYONAKA LOLITA
CARAMEL KISS
Bride of the water god
Love Monster
Meru Puri
MAYONAKA WA DAME YO

I suggest Detective Conan or Asatte no Houkou.

Also I found a list of mangas that are in the category 'Old to young'. I don't know if these will help, but I hope they do.^_^:

[url]http://www.mangaupdates.com/series.html?category=Old+to+Young[/url]

Also Shibatora

SHOUNEN: Nanatsu no Taizai

Immortal Rain should fit, he's the antagonist though...

in need of romance?
uncommon, sad, sweet

Heptagon and Haruka na Machi e and maybe the first story in Soko wa nemuri no mori.

Sakuranbo Syndrome fits the bill the most I think. It's kind of mature though.

omoide emanon
And perhaps hajimari no niina?

I second Detective Conan (is a given), Meru Puri (feels a bit wrong to me, but whatever), Matantei Loki, and Kazan (awesome story).

A character, who almost fits the full criteria, exists in Platinum Garden. He's not a main main character, but an important character, nonetheless. He does mingle with his peers, but when I say "peers", I don't mean little kids. Poor guy. Forever stuck until.....

Next, these two manga will have their names, along with the info, put into spoiler tags. Both of these manga have people, who don't know their bodies are actually older than they're suppose to be. ^^; Thus, everyone around them believe they're at that age. (Main character ages are currently around 15-16, so yes, high school age. Of course, this all started even before they got to those ages....)

• Their manga tags are off so their titles are hidden. They're popular enough to be found...

D.Gray-Man - The main character de-aged for unknown reasons, as of this moment. So, he actually believes his own current age because he doesn't remember his past. On top of that, he has a soul of another person within him, who knows his past.

Pandora Hearts - The main character's body isn't actually his, but someone else's. His soul just ended up being placed into that body due to a past event.

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