This used to be the #1 most frequently asked question at Cool Nurse. A reader sent in some answers. I thought this was worth putting on the site, but I do not know where he got it from. It might be from an Internet survey and may not be that accurate, it has been on this site since 2001.
These statistics are not clinical measurements, they are the average results of people who answered an Internet poll. If you know you are a late maturer, then the results for your age group will be meaningless -- you need to look at a younger age group. Don't be too concerned if you fall well outside the measurements for your age group. The penis is an organ that varies a lot from male to male - there is no “right” or “wrong” when it comes to penis size.
Measured along the top, how long is your erect penis?
By age, the averages found were:
Age
Size
12 4.1" (104 mm)
13 5.1" (130 mm)
14 6.0" (152 mm)
15 6.1" (155 mm)
16 6.3" (160 mm)
17 6.5" (165 mm)
18 6.4" (163 mm)
19 6.2" (157 mm)
20 6.2" (157 mm)
Conclusions:
The average adult penis size is about 6.2 inches or (157mm). It is interesting that in this study, penis size 'peaks' at age 17. Since penises do not get smaller as a male approaches age 21, it can be concluded that boys aged 16, 17, and 18 are most likely to exaggerate the size of their penises. Just a guess. (Several physicians suggested that the average penis size was about 6 inches, but they weren't measuring.) So, an average of 6.2 seems incorrect and maybe guys taking the survey exaggerated.