A lot of older couples do stop having as much sex or even any sex at all.
In men, testosterone levels drop significantly. They start dropping at age 30-31 on average. The more men age, the lower it gets. One way to reduce the drop is vigorous exercise, but that will only reduce the rate not eliminate it.
In women, estrogen levels drop significantly, marked most significantly by menopause. I don't know what precise age that is; 40's I think. I don't think vigorous exercise helps in women.
Both hormones affect the sex drive in the respective genders.
If you are a male and have sex daily (or masturbate), try to quit for 5 days and you'll notice at the end the only thing going through your mind will be SEX SEX SEX. That's testosterone building up.