Is it true that when we see 伯母 it usually (90%) means "parent's elder sister" and sometimes (10%) can be used to refer to the parent's younger sister?
On the contrary, when we see 叔母 does it always (100%) mean "parent's younger sister" and never "parent's elder sister" ?
So far as I know, 伯母 does not ever intentionally mean anything other than "elder" aunt.
The issue you are probably seeing is that since the words are pronounced the same, and that these kinds of age differentiations are not as important as they once might have been, the distinction between the two words is something that Japanese can mix up.
I saw a Japanese TV show one time where they had two men, one with a shirt that had 叔父【おじ】(younger uncle) written on it, and the other with 伯父【おじ】(older uncle) written on it, and people were tested on which was which. About half got it wrong.
I just asked some random people beside me at the coffee shop, and one woman said that the difference was not to do with age at all, but whether or not the aunt or uncle was related by blood or marriage. Huh. The dictionary clearly says it's about age, so I take this as further indication that the usage and meanings are becoming less clear in modern Japanese.
And a little internet searching further shows that Japanese can be unclear on the difference.
So, bottom line, no, it's not the case that 伯母 can ever intentionally be used to mean the younger aunt, but it probably sometimes is mistakenly used that way.
In Chinese -- which the Japanese kanji are derived from and modelled after -- it's a title that a person would use to refer to the wife of their father's younger brother.
Still an aunt (regardless of age, because if she's older than your father but married his younger brother --- she gets this "demoted" form of paternal aunt-in-law).
For "auntie" -- in general... like someone who is not blood related, nor related by marriage, but is just a friend of your parents or whatnot you can use the 姨 word that is given to a maternal aunt (your mother's sister).
