The penis typically grows for about five to six years during puberty, starting around age 11 or 12 and reaching its final size by age 17 or 18. The exact timeline varies from person to person, but growth follows a predictable pattern that tracks closely with the overall puberty growth spurt.
When Growth Starts and Stops
Penile growth doesn’t begin right when puberty does. The first visible sign of puberty in boys is growth of the testicles and scrotum, which typically starts between ages 9 and 14. The penis begins growing about six months later, at a mean age of 11.5 years. From there, growth continues through two distinct phases before finishing.
Between ages 10 and 16, the penis grows in both length and girth, with growth accelerating in the middle of that window. The fastest rate of penile growth occurs around age 13.5, which happens to be the same age most boys hit their peak height growth. So if you notice a major height spurt, penile growth is likely at or near its peak as well. By age 16 or 17, most boys have completed puberty entirely, and penile growth has stopped or nearly stopped. Some individuals continue developing slightly until 18, but significant changes after that point are uncommon.
What Drives Growth (and What Stops It)
Two hormones do the heavy lifting. Testosterone, produced in large quantities once the testicles mature, is responsible for many changes during puberty, including muscle development and voice deepening. A related hormone called DHT (a more potent form of testosterone) plays a specific role in the development of external genitalia, both during fetal development and again at puberty when it promotes further growth of the penis and scrotum.
Interestingly, growth doesn’t stop because hormone levels drop. Testosterone and DHT levels remain high, or even continue to rise, after the penis reaches its adult size. What actually happens is that the tissue itself becomes less responsive. The cells in penile tissue gradually reduce the number of hormone receptors they carry, a process called downregulation. Once enough receptors have been lost, the tissue can no longer respond to hormonal signals with new growth, even though those signals are still present. This built-in biological limit is why the penis reaches a set size and stays there.
The Stages of Development
Doctors use a five-stage framework to track puberty progression. Understanding where growth falls within these stages can help put the timeline in perspective.
- Stage 2 (ages 9 to 14): The testicles and scrotum begin to enlarge. The penis hasn’t started growing noticeably yet.
- Stage 3 (ages 10 to 16): The penis begins to grow, primarily in length. Pubic hair starts appearing.
- Stage 4 (ages 11 to 16): Growth continues in both length and girth. The skin of the scrotum darkens. This is typically when the most noticeable changes occur.
- Stage 5 (around age 16 to 17): Growth is complete. The genitals have reached their adult size and shape.
These age ranges overlap because boys enter and move through puberty at different speeds. A boy who starts puberty at 10 will likely finish earlier than one who starts at 13, but both will progress through the same stages in the same order. Starting puberty later does not mean ending up with a smaller adult size.
Average Adult Size
A large-scale review analyzing 17 studies and over 15,500 men found that the average erect length is 13.12 cm, or about 5.16 inches. All measurements in that review were taken by health professionals using a standardized method, which makes the data more reliable than self-reported surveys, which tend to skew higher.
There is natural variation around that average. Most men fall within roughly an inch above or below it. Size is largely determined by genetics, and no supplement, exercise, or food has been shown to meaningfully change the outcome once growth is complete. The same receptor downregulation that stops growth during late puberty also means that adding more testosterone afterward won’t restart it.
Why Growth Timelines Vary
The biggest factor in when growth starts and finishes is when puberty begins, and that’s mostly genetic. Nutrition and overall health play a role too. Boys who experience chronic malnutrition or certain hormonal conditions may see delayed puberty, which can push the entire growth timeline later. In most cases, late bloomers still reach the same developmental endpoints; they just arrive there on a different schedule.
Conditions that affect hormone production or hormone sensitivity are less common but can have a more meaningful impact. Boys with very low DHT levels (due to a genetic enzyme deficiency) still experience penile growth from testosterone alone, but the process may look different. For the vast majority of boys, though, puberty proceeds through all five stages without intervention, and growth wraps up by the late teens.