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.