The growth of the penis during puberty usually ceases as puberty does. On average this is between the ages of 15 and 17, however can range between 14 and 20. 21 is considered to be the average age where puberty and growth fully stops.
The average erect penis length of a teenager is considered to be between 5 and 6 inches and between 4.3 and 4.5 inches in girth. An adult's erect penis is usually between 5 and 6.5 inches in length and approximately 4.5 inches in girth.
A flaccid penis can vary in size usually between an average of 3-4.5 inches. Other factors may affect this, such as cold temperature and exhaustion causing the flaccid penis to become temporarily smaller. There’s no correlation between a flaccid penis and an erect one; however, if a flaccid penis were to be stretched (pulled) this length is considered to be the size of the penis when erect.
There are other factors that can affect penis growth. A commonly believed myth is that there’s a correlation between race and penis size, however there is no scientific evidence or case study to support this. Other factors that affect the growth of the penis include blood flow, diet and hormones.