Many relationships go through ups and downs the longer you have been together, including sex drive. The child rearing years can be fraught because time and money are usually in short supply. My husband often worked seven days a week and I had a full time job too.
Women of my generation tended to have children younger than women of today, so the children were grown up when I was in my forties. Some women then begin to doubt their role in life after the kids have left home (didn't affect me) especially if the relationship with their partner isn't sound. Then comes the menopause and that's a whole nother kettle of fish, as they say! Some women sail through it, I did not! The hormonal dips and hot flushes were bad enough but I also had hormonal-induced rages, not caused by anyone in particular but usually directed at the first person in the firing line - more often than not my poor husband. Believe me, that's not conducive to a good sex life. I also had extreme vaginal dryness, constant bladder infections etc. When it's over it's fantastic though. There is no need for contraception and the fear of pregnancy is gone.
We are both in our sixties and have a fantastic sex life. We have sex at least once a day, usually more! We are much more adventurous now than in our younger days and a lot more open with each other. As my much older relative says, sex is wasted on the young