The penis typically starts to grow between ages 9 and 14, which is when male puberty begins. For most boys, noticeable penile growth kicks in a bit after the first signs of puberty appear, since the testicles usually enlarge first. The entire process from start to adult size takes roughly four to five years.
What Happens First
Puberty doesn’t begin with penile growth. The first visible change is testicular enlargement, along with some thinning and reddening of the scrotum. This can start as early as age 9 or as late as 14. Many boys (and their parents) don’t even notice this initial stage because the changes are subtle.
Once the testicles begin to grow, they ramp up production of testosterone. That surge in testosterone is what drives penile growth, along with other familiar changes like body hair, a deeper voice, and growth spurts. The penis typically begins lengthening within a year or so of the testicles starting to enlarge.
How Growth Progresses
Penile growth doesn’t happen all at once. It follows a general pattern that doctors describe in stages:
- Early puberty (roughly ages 9 to 12): The testicles and scrotum start growing. The penis hasn’t changed much yet.
- Mid-puberty (roughly ages 12 to 14): The penis begins to grow, mostly in length at first. Pubic hair appears and becomes coarser. Height is increasing noticeably.
- Later puberty (roughly ages 14 to 16): The penis continues growing in both length and girth. This stage overlaps with the peak height growth spurt, which averages about 10 centimeters (roughly 4 inches) per year around age 13 to 14. Voice changes typically happen around age 14, and acne often follows a few months later.
- Completion (roughly ages 16 to 18): The penis reaches its mature adult size. Most boys hit their final size by about age 16 or 17, though some continued subtle growth is possible into the late teens or very early 20s.
These age ranges are averages. A boy who starts puberty at 10 will likely finish earlier than one who starts at 14. The total growth period typically lasts four to five years from when it begins.
Why Testosterone Drives the Process
Testosterone and its more potent form (produced naturally in the body) act on receptors in penile tissue, stimulating the cells to multiply and the tissue to expand. Growth hormone works alongside testosterone in this process. Research in endocrinology has shown that growth hormone boosts the number of receptors that testosterone can bind to, essentially making the tissue more responsive to the hormonal signal. This is why both hormones need to be present at adequate levels for normal development.
This hormonal interplay also explains why penile growth tracks closely with other testosterone-driven changes. If you’re seeing new body hair, a deepening voice, or a growth spurt in height, those are signs that the same hormones responsible for genital development are active.
What Affects the Timing
The single biggest factor is genetics. If a boy’s father or older brothers went through puberty on the earlier or later side, he’s likely to follow a similar timeline. Ethnicity and family background also play a role in when the process starts.
Beyond genetics, several other factors can shift the timing earlier or later:
- Nutrition: Boys who are well-nourished tend to start puberty earlier. Restricting calories, whether from food insecurity or intentional dieting, can delay the onset.
- Body weight: Higher body fat is associated with earlier activation of the hormonal signals that trigger puberty. Over the past 50 years, the average age of puberty onset has trended slightly earlier worldwide, partly attributed to improved nutrition and rising rates of childhood obesity.
- Chronic illness: Conditions that affect overall health or energy balance can delay puberty by suppressing the hormonal cascade that starts the process.
- Stress and mental health: Depression, significant psychosocial stress, and intense physical training (like elite-level sports) can also push the start of puberty later.
- Environmental chemicals: Exposure to certain endocrine-disrupting chemicals has been linked to shifts in pubertal timing, though the effects vary.
When Development Might Be Delayed
Delayed puberty in boys is formally defined as no signs of puberty by age 14. Specific markers doctors look for include testicles smaller than about one inch at age 14, a penis that remains small and immature-looking by age 13, and very little or no body hair by age 15. A voice that stays high-pitched and a body that remains short and thin for age are also considered signs.
The most common cause is called constitutional delay, which simply means a boy is a “late bloomer.” It runs in families and isn’t a medical problem. These boys eventually go through puberty on their own, just on a later schedule. Less commonly, delayed puberty can result from hormonal conditions, genetic factors, chronic illness, or poor nutrition. When the underlying cause is identified and addressed, whether that means treating an illness or improving nutritional status, puberty typically progresses normally.
What “Done Growing” Actually Means
Most boys reach their mature genital size by around age 16 to 17. By 18 or 19, significant additional growth in length or girth is unlikely. Some sources note that very minor changes can continue into the early 20s, but these are subtle enough that most people wouldn’t notice them.
It’s worth knowing that the pace of development varies enormously among boys the same age. Two 13-year-olds can look completely different in terms of physical maturity, and both can be entirely normal. A boy who feels like he’s “behind” compared to peers may simply be on a later timetable that will catch up over the next year or two.