Dirty Kid

Near opposite of the Dirty Old Man, where a kid is ridiculously young, but still has some signs of... "interest".

What's most notable is that often, kids or babies are allowed to cop a feel, simply because it's said that they don't understand what they're doing.

See also Kidanova, which deals more with the romantic aspects, as opposed to the sexual ones.

Not to be confused with The Pig Pen, who is filthy in a different way.

Examples:

Anime and Manga

Johji from the Death-T arc in Yu-Gi-Oh.
Shin from Shin Chan.
Momoka from Mawaru Penguindrum. She went missing as an elementary schooler. Her Diary is very detailed and specific about how her future life would have gone. This includes sexual activities with her prospect Victorious Childhood Friend, Tabuki.

Baby Herman from Who Framed Roger Rabbit. He complains, at one point, about having "a fifty-year-old lust, and a three-year-old dinky."
- But Baby Herman is really that old, he just looks like a baby.
- The character he plays as in the shorts may have elements of this, though, with Roger finding an adult magazine (complete with centerfold of Jessica) in his carriage at some point.
In telling his origin story, Don Juan Demarco claimed to have started out this way (but he was lying).

Robert Heinlein's Stranger in A Strange Land. When he first arrives on Earth, Valentine Michael Smith is a child in an adult's body, knowing nothing of human culture. While he's being bathed by a nurse he innocently touches her breast.

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Lily of How I Met Your Mother is a kindergarten teacher; in the pilot episode she comes home with a painty handprint on her (white) shirt. Three guesses where.