Age of onset of menopause is also influenced/associated (somehow) by birth month. My birth month of October rather perfectly fit their findings. https://www.ncbi.nlm.nih.gov/pubmed/15890738
"Age at menopause of 2822 post-menopausal women (>12 months of amenorrhoea) was stratified by month and season of birth. Mean age at menopause was 49.42 years (SEM: 0.78 years). Menopause occurred earlier for women born in the spring (age 49.04+/-0.15 years) than in the autumn (49.97+/-0.14 years). The earliest menopause was found in women born in March (48.9+/-0.25 years) and the latest in women born in October (50.3+/-0.25 years). The effect of season of birth on age at menopause remained even when considering factors that in our analysis were capable of significantly interfering with the timing of menopause, such as age at menarche, body mass index, smoking habit, level of education and type of job."
FWIW, I had pretty much a non-issue menopause -- some hot flashes, but nothing like most of my friends went through. They were generally years younger than I was at the time they started -- maybe it was a function of going down a steep hill in their case vs. a more gentle decline in mine? Never took HRT.