Women are most fertile from their late teens to late twenties. After 30, fertility declines more rapidly. By age 36, the chance of conceiving each month drops to about half compared to early 20s, and by age 41, it decreases to just 4% per month.

This decline is due to a decrease in the number and quality of eggs. Women are born with about 2 million eggs, which reduce to around 400,000 by puberty, and continue to decline.

Men's fertility also diminishes with age, though less drastically than women's. Male factors account for over 40% of infertility cases, often influenced by sperm abnormalities and reduced sperm counts as they age.