The medical term for painful sex is dysparunia. And it can be a chronic or an acute condition; it can occur in young women as well as old. Fortunately, many cases are due to newly aquired infections or conditions that can be cured.
Some women have non-physicial causes, but in general it's more often seen in older women than in younger women. As women age into the menopause the vaginal lining thins and sex can become more uncomfortable.
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But there are many factors that cause painful sex in menopause, and estrogen in some studies only cures about 75% of cases. Why there are persistent problems in some cases can be complex: pelvic floor problems, weakened muscles, too little intercourse, not enough estrogen can be tolerated, secondary effects of the treatment itself.