Men have a biological clock too—it just runs slower

The sex-specific changes in women as they age are well known:
declining fertility and an increased chance of birth defects, and
health risks caused by hormonal changes. Less well known is that
similar changes oc-cur in men as they age.

Declining fertility
Men over 35 are twice as likely to be infertile as men younger than 25.
Couples in which the man is older than 35 conceive a child about half
as often as couples in which the man is 30 or younger. Even couples
undergoing fertility treatments using artificial insemination take
longer to achieve pregnancy when the male partner is older.

The genetic quality of a man’s sperm, as indicated by the
incidence of birth defects in his offspring, also declines with age.
For example, men who are 40 or older are almost 6 times more likely to
have a child with autism than men younger than 30. (The mother’s age
doesn’t seem to play a role in autism risk.) Similarly, men older than
40 are more than twice as likely as men in their 20s to have a child
with schizophrenia.

Lower testosterone has a broad impact
After age 30, a man’s testosterone level begins to fall. Men don’t
experience the steep hormonal drop that women do with estrogen at
menopause. Rather, men’s testosterone level declines about 1% per year.
The threshold at which problems arise varies widely. But reduced
testosterone is associated with decreased muscle mass, bone density,
energy, and libido, and increased fat and abdominal obesity, insulin
resistance (which leads to rising blood sugar levels), irritability,
and depression. An 8-year study of men over 40 found the death rate
almost twice as high among those with abnormally low testosterone
levels as among those with normal levels.

Low testosterone levels are strongly associated with metabolic
syndrome (which includes such factors as high blood pressure, high
triglycerides, and insulin resistance) as well as diabetes, with its
added risks of heart, nerve, and kidney problems. Low testosterone in
men with type 2 diabetes is associated with a higher incidence of
erectile dysfunction (impotence).  

Erectile dysfunction is also associated with other conditions
that become more prevalent with age: heart disease, depression, and an
enlarged prostate. All of these problems tend to influence one another.
For example, erectile dysfunction increases the likelihood of
developing depression, and, in turn, depression can contribute to heart
disease.