I've seen a few of mangas with a character that at first looks like a kid or tween but is actually older than you think. (Like, looks 16 is actually 26) I'm particularly interested in this being important to the plot of the story. Like a 30 year old guy that looks like a high schooler and falls in love with an actual high schooler so he poses as such just to stayed with him. Or a 25 year old is believed to be a lost kid who is 10 years old and instead of saying who he really is he plays along. Some examples of characters like this are:

Hanyuu-sensei no Ryou Seikatsu
Junjou Bitch, Hatsukoi Kei - Chapter 4
Kirepapa
Kuzu Seito to Stalker-sensei

Even Mitsukuni "Honey" Haninozuka from Ouran High School Host Clubeven though is not yaio (which is a shame)