How large should a penis be? What's the average penis size? Is my penis normal? When does the penis stop growing, and what happens when you get to middle age? These are questions it seems every guy is interested in. Giving into popular demand, here are the answers you've been looking for — because we're taking a look at the penis from childhood to adulthood.
What Happens To The Penis From The Womb To Puberty?
While a baby's chromosomes are determined at conception, the processes that set genital development in motion do not start until about the seventh week of pregnancy. Under the influence of hormones, including testosterone and anti-Mullerian hormone, baby boys begin developing their genitals.  The growth of the penis is fastest between 13 and 24 weeks gestation , and penile development has even been studied as a means to determine the correct gestational age. It has been found, for instance, that mean penile length is 3.88 millimeters at 14 weeks gestation, and 23.77 millimeters at 38 weeks gestation, when the baby is just about ready to be born. 
Other interesting things also happen in the womb. The testicles, which develop in the abdomen, descend into the scrotum before birth. When this doesn't happen, the baby is born with one or both testes still undescended. This situation may resolve itself within a year, or surgery may be required. 
In uncircumcised boys, the foreskin will typically be fused to the glans, and it will likely remain unretractable until the boy is between three and five years of age, though it can also take much longer, even until the onset or end of puberty. This is normal, but those parents who are concerned should always consult a physician who is familiar with uncircumcised penises if they are concerned, rather than trying to forcibly retract the foreskin. 
Pubertal Penile Growth: What To Expect?
Research shows that the bulk of both penile and testicular development tends to take place between the ages of 12 and 16. Since many reading this piece will be here because they want to know whether their penis is normal or not, we'll give you some numbers from an in-depth study into the matter. What is the average penile length during the ages you're most likely to be interested in this? Let's take a look:
- 12 year old boys have a mean penis length of 5.86 centimeters, with a circumference of 6.59 cm.
- 13 year old boys have a mean penis length of 7.11 centimeters, with a circumference of 7.70 cm.
- 14 year old boys have a mean penis length of 8.04 centimeters, with a circumference of 8.65 cm.
- 15 year old boys have a mean penis length of 8.69 centimeters, with a circumference of 9.10 cm.
- 16 year old boys have a mean penis length of 9.01 centimeters, with a circumference of 9.26 cm.
Any boy who hasn't shown signs of puberty, or believes his penile development is lacking, can never go wrong in consulting a medical professional, even if it's just to be reassured that everything is fine.
Adulthood: Is My Penis Normal?
Probably. As one research paper said:
"The reported normal length and girth of an adult flaccid penis ranges between 7.6 cm and 13.0 cm in length and 8.5 cm and 10.5 cm in circumference, and the reported normal length and girth of an erect penis ranges between 12.7 cm and 17.7 cm in length and 11.3 cm and 13.0 cm in circumference." 
The same research paper reports that the vast majority of men who approach doctors about the possibility of penis enlargement fall well within these normal ranges. The moral of the story is that it's quite easy to feel inadequate about your penis even when it's statistically perfectly normal.
As middle age sets in and testosterone levels decline, it is also quite possible that you will see a gradual reduction in the size of your penis and scrotum, as well as a loss of libido.  In addition, any man who has gained a significant amount of weight may be under the impression that his penis has decreased in size, even though it hasn't, because fat tissue covers a portion of the penis.