What are the connotations of "Obasan"

[mmmason8967]

As a beginner I've learned that "obasan" means "aunt" and from watching Japanese movies I've also learned that it can be used to address a woman with a meaning something like "lady" or "madam".

There is a kind of set joke that crops up in a small number of the anime I've seen that involves a small child addressing one of the female leads as "obasan" to the evident annoyance of said female lead. Since the female lead is usually in the 18 - 25 age range, I'm guessing that obasan implies either an age considerably greater than 25 or, possibly, some kind of frumpy-ness or staidness. It certainly seems to imply something that an 18-25 year old female doesn't like to have applied to them.

So what are the connotations of "obasan"?

[Jessi]

mmmason,
You're pretty much spot on! It's kind of like calling people "ma'am" in English... some people don't like it because it they feel that they're not "old enough" to be referred to as ma'am I think it's the same thing - mostly an age issue.

[mmmason8967]

It's kind of like calling people "ma'am" in English...

Aaaaaaaaaah, I see--the small child isn't using a poorly-chosen word but is actually being polite and quite proper. And that's why the female lead looks so put out but never says anything...