Low desire is not a character flaw and it is not simply age. It usually has a cause.
It affects both sexes
In men, testosterone is the most common hormonal driver, but it is far from the only one. In women, testosterone matters too and has usually been falling since the late twenties, alongside the estrogen changes of perimenopause and menopause.
Desire is generated centrally, in the brain, which is why it responds to sleep, stress, mood and medication as much as to hormones.
The causes worth ruling out
Low testosterone in either sex. Thyroid disease. Depression, and just as often the antidepressants used to treat it, since SSRIs are among the most common causes of lost desire. Chronic sleep deprivation. Heavy alcohol use. Opioids. Chronic illness and pain.
Relationship strain and life stress are real contributors rather than a diagnosis of exclusion, and they frequently sit alongside a hormonal cause rather than instead of one.
In women specifically
Pain with sex is a common and under-asked cause. Desire reliably falls when sex has become uncomfortable, and treating the discomfort often restores it without anything else being needed.
That discomfort is usually genitourinary syndrome of menopause, and it is very treatable.
What treatment looks like
The order of operations matters more than any single prescription. Get the hormones evaluated properly and correct what is correctable. Review the medication list, because the answer is often sitting in it. Treat pain if pain is present, because desire does not survive dreading the act. Where desire persists after all of that, centrally acting options target the desire pathway itself: the peptide PT-141, FDA approved as Vyleesi, and the daily oral option Addyi. Low desire has more real treatments today than it has ever had.