How Nails Grow
Your nails grow from a area called the nail matrix, which sits underneath the skin at the base of your nail. The cells in this matrix are constantly dividing and multiplying. As new cells form, they push the older cells forward and outward, which creates the growth you see. The nail itself is made of a tough protein called keratin, which is also found in your hair and skin.
Why Your Body Does This
Nails grow throughout your life because your body continuously replaces old cells with new ones. This is a normal biological process. Your nails protect the sensitive tips of your fingers and toes, so your body keeps producing new nail material to replace any that wears away or breaks off. This growth process is controlled by your genes and your overall health.
Factors That Affect Growth Speed
Your nail growth rate depends on several things. Age, health, nutrition, and hormones all play a role. Nails typically grow faster in young, healthy people and slower in older adults. They also grow faster in warm weather and during pregnancy. If you are sick or not eating well, your nails may grow more slowly. Men's nails usually grow slightly faster than women's nails.
Why Nails Stop at a Certain Length
Although nails grow continuously, they eventually stop getting longer because they naturally break, wear down, or you trim them. Unlike hair, which also grows from a root, nails have a fixed growth pattern. Once a nail cell is pushed far enough from the matrix, it no longer receives nutrients and cannot grow thicker. The nail at the end of your finger is actually made of dead cells. 
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