The pettanko is a flat-chested female character.
This might actually bother her if she is already considered childish and prone to tantrums, so the sore spot becomes endearing. Occasionally, she is proud of that flat chest. However, there is a difference between merely being flat-chested and actually being underage. Nonetheless, it is very useful in creating a character who is Older than They Look.
Note: This is a fan-speak term, not a trope. Do not use it as such. Doesn't mean it can't still be used, but just not as a trope.