|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...
Spoiler (mouse over to view)
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.
Looking for memory related manga/manhwa? -> click me