For most males, the penis reaches its full size between ages 16 and 21, with the majority of growth finishing by around 18 or 19. The exact timeline depends on when puberty starts, how quickly it progresses, and individual genetics. By your early twenties, any remaining growth is minimal to nonexistent.
How Growth Progresses During Puberty
Penile growth follows a predictable sequence tied to puberty’s stages. The first visible sign of puberty in boys is growth of the testicles, which typically begins between ages 9 and 14. The penis itself starts growing shortly after, usually during what doctors call the middle stages of puberty, roughly between ages 10 and 16. During this window, the penis grows in both length and girth, and the skin of the scrotum darkens.
The entire process from the first signs of puberty to full physical maturity takes about 3.5 to 4 years. So a boy who starts puberty at 10 may finish growing by 14, while one who starts at 14 could still be developing at 18. This is why there’s such a wide range in the “final” age. The penis can actually continue to develop for one to two years after height growth stops, so even after you’ve hit your adult height, some additional change is possible.
What Drives the Growth
Testosterone is the primary hormone behind male puberty, but a more potent form of it, called DHT, plays the central role in genital development specifically. DHT is what drives the penis and scrotum to grow larger during puberty. It’s the same hormone responsible for forming male external genitalia before birth. Once puberty winds down and hormone levels stabilize at adult levels, the signal for tissue expansion essentially stops, and growth plateaus.
Does Anything Change After Your Early Twenties?
By age 18 or 19, the penis is unlikely to grow much longer or thicker. Some very minor changes may continue into the early twenties, but these are small enough that most people wouldn’t notice a difference. After that point, the size you have is the size you’ll keep.
That said, the penis can appear to change size over time due to factors that aren’t actual growth. Weight gain adds fat to the area above the base of the penis, which can make it look shorter. Weight loss reverses this effect. Changes in blood flow from cardiovascular fitness, smoking, or aging also affect erection quality, which can influence perceived size without changing the underlying anatomy.
Average Adult Size
A large study of over 15,000 men found that the average erect penis is 5.1 inches long with a circumference of 4.5 inches. Flaccid, the average length is 3.6 inches and the average girth is 3.7 inches. There’s a wide range of normal on either side of these numbers.
If you’re comparing yourself to these averages, keep in mind that how you measure matters. Medical guidelines call for measuring along the top of the penis from the base (pressed against the pubic bone) to the tip of the head. Measuring from the side, the underside, or without pressing to the bone gives a shorter and less standardized number. The pubic bone reference point is used because it removes the variable of body fat, giving a consistent measurement regardless of weight.
What If Puberty Seems Late
Some boys don’t show signs of puberty until closer to 14, which can feel concerning but is usually just a family trait. This is called constitutional delayed puberty, and it’s the most common reason for late development. Boys in this category are sometimes called “late bloomers,” and once puberty kicks in, it progresses normally and reaches the same endpoint.
Signs that puberty may be genuinely delayed include testicles that remain smaller than about one inch by age 14, a penis that still looks prepubescent by age 13, very little body hair by age 15, or a voice that hasn’t begun to deepen. If several of these are present, it’s worth having a doctor check hormone levels. In most cases, the body catches up on its own. When it doesn’t, hormone therapy can jump-start the process and development proceeds from there.
The key point for late bloomers: starting puberty later doesn’t mean ending up with a smaller adult size. It just means the growth timeline is shifted. A boy who begins puberty at 14 may not finish developing until 18 or 19, while an early starter might be done by 15 or 16. The final result is determined more by genetics than by when the process begins.