Some of the changes of aging start as early as the third decade of life. After age 25–30, for example, the average man's maximum attainable heart rate declines by about one beat per minute, per year, and his heart's peak capacity to pump blood drifts down by 5%–10% per decade. That's why a healthy 25-year-old heart can pump 2½ quarts of blood a minute, but a 65-year-old heart can't get above 1½ quarts, and an 80-year-old heart can pump only about a quart, even if it's disease-free. In everyday terms, this diminished aerobic capacity can produce fatigue and breathlessness with modest daily activities.
Starting in middle age, a man's blood vessels begin to stiffen and his blood pressure often creeps up as well. His blood itself changes, becoming more viscous (thicker and stickier) and harder to pump through the body, even though the number of oxygen-carrying red blood cells declines.
Most Americans begin to gain weight in midlife, putting on 3–4 pounds a year. But since men start to lose muscle in their 40s, that extra weight is all fat. This extra fat contributes to a rise in LDL ("bad") cholesterol and a fall in HDL ("good") cholesterol. It also helps explain why blood sugar levels rise by about 6 points per decade, making type 2 diabetes distressingly common in senior citizens.