Age-related decline in fertility refers to the decrease in reproductive capabilities as individuals grow older. In women, this decline typically begins in their late 20s to early 30s, with a more significant drop after age 35. This is primarily due to a decrease in the quantity and quality of eggs available for fertilization. For men, fertility also declines with age, though the changes are generally more gradual. Factors such as reduced sperm quality and lower testosterone levels can affect male fertility, particularly after age 40. Overall, both men and women may face challenges in conceiving as they age.