The penis typically stops growing by the end of puberty, which generally occurs between the ages of 18 and 21. Most significant growth happens during puberty, starting around age 9 to 14 and slowing down after the mid-teen years. Some growth can continue into the early 20s, but by about 18 to 21 years, the penis usually reaches its adult size. Growth rates and timing vary by individual due to genetics and hormonal factors, especially testosterone levels during puberty. After this period, the penis does not grow further under normal conditions.