The Sea Wasp (Ryk E. Spoor)

[1] burger_eater's original post posited that the kidling will remain 5 for eternity or until staked and be unable to ever learn to care for herself. I think that changes things a lot (enough that I know what my hypothetical "if none of these extenuating circumstances are true" answer would be), but the setup still needs more explanation to elicit a proper answer.

Actually, there was a character in the Wild Cards novels ("Sprout", I think) who was exactly like that, just nonvampiric. (Eternally five, never growing physically or mentally.)