In the Black Jack TV series, Pinoco (of course) claims to be 18. But in the last episode of Black Jack 21, which supposedly takes place at least a year later, Pinoco again claims to be 18.
Do you think the idea of a birthday just never occurs to her, or is she purposely fixated on that age? I ask because in the Black Jack episode of Astro Boy, Pinoco claimed to have "just turned twenty-one", but that might have been a dub edit. (Plus, that was obviously a different continuity and might not even count)
Personally, I don't think she does it on purpose. Since she doesn't actually age, birthdays don't cross her mind, so she just keeps saying she's 18. What do you think?