Hmmm... there's two ways to interpret that question, and the second answer is 'About the time senility kicks in...' but if you're asking at what age your penis will stop increasing in size due to puberty, the answer is that it is likely to have reached it's maximum size by about age 15/16. That's a rough guide of course. If for any reason you're concerned about your penis size bear in mind the following: 1) Any man who worries or boasts about the size of his flacid penis is an idiot. 2) Most men will lie about the size of their penis when asked. An American survey (it might have been Shere Hite or it might have been Kinsey) showed that most men over estimated the size of their erect penis by between 1-1.5 inches. This is becuase they measure from halfway up their backs instead of along the top. Unfortunately, this doesn't make it easy to tell truth, becuase your friends will simply think you're trying to justify your 'small' winkie. 3) Most women are perfectly happy with an 'average' sized penis, provided their lover is halfway competant. Gay men tend to be more competitive about size. 4) If your penis is abnormally small (a micro-penis) it can be fixed with surgery.