I'm looking for manga where a main character is stuck as a child. Where they've aged mentally but never grow up. I'm interested in the concept of them dealing with those around them aging and them not being able to.

I'm Not looking for manga where the main character looks like a kid but is an adult.

The best examples I can think of are stories involving vampires, like dance in the vampire bund or the girl from the movie Interview with a vampire.

Feed your head.

Besides the obvious Detective Conan, there's also:
Baccano!
Demon Seiten kinda...not sure if their mental state rejuvenates as well.
The Bride of the Water God But he transforms to an adult during the night. D=

[img]http://28.media.tumblr.com/tumblr_lq2hczRc5d1r1rvcno1_500.gif[/img]
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Can the OP clarify themselves? D:

Are adults that somehow wind up in a child's body okay? (like Detective Conan), or are you talking about people who never manage to mature beyond their child form because of some disorder or whatever?

If it's the latter, try the Higurashi franchise then. 😉 Furude Rika is a girl that has matured mentally a lot more than her peers despite being still a child in appearance.

(she lived for hundreds of years, technically)

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I'm not looking for manga where an adult/teen is turned into a child (like in conan). The "wise beyond their years" aspect of it is important.

Feed your head.

Ichinensei ni nacchattara -- a teenage boy is 'converted' to a first-grade girl.

Blood Alone
Soko wa Nemuri no Mori

I second Blood Alone.
Chrno Crusade this should fit ok.
Ookami to Koushinryou I think Horo could be counted in, unless you think 15 years old is too much. She's also very wise.
Ubel Blatt but honestly I don't remember/know how really old is, so maybe I'm wrong.

What I like in Trivial Pursuit style. Pick your category:[img]http://i379.photobucket.com/albums/oo233/Reid4891/Manga/trivia-1.jpg[/img]

Detective Conan

Also, one of Takahashi's Mermaid stories is about a boy who became immortal at the age of 9(?) or so and had to deal with it for several hundred years. I think it's "Mermaid's Scar."

I think the one you're looking for is Asatte no Houkou. It's about a child trapped in an adult body and a very realist adult trapped in a child's body (no they didn't swap body, they just aged up/down)

mushi to medama to teddy bear ^(o.o)^ a girl turns immortal and an already immortal girl lived for 1000 or more years?

[img]http://i718.photobucket.com/albums/ww187/D_dragon/omfgpudding-1.jpg[/img]

Shibariya Komachi

[img]http://i198.photobucket.com/albums/aa189/LilArusha/b38e1ca1f025f0_main.png[/img]

Ubel Blatt does fit.

There's also the novel Interview with the Vampire by Anne Rice that involves this from what I remember. (I just reread your first post, so I now know you already know about this. >>)

kazan

Taboo Tattoo