I just heard on the radio (female-lead talkshow, female doctor present, just to clarify the bias) that women start losing their cognitive abilities during menopause, some even before.
Statistically, women in this stage of life will lose 5% of their ability in 10 years, this is measurable. Degradation includes "brain fog", slowness of thought and loss of vocabulary. For those who are overweight, the process may start in their late 30s, while those who are thin/exercise may mitigate it a bit.

So there you have it: according to Science, women over 40 are already starting to become demented, it's not our imagination.