It is difficult to know exactly when your penis will stop growing. During puberty, a young man’s testicles begin to grow larger, followed by the penis. On average the penis will reach its final size about 4-6 years after the testicles enlarge. Most guys are fully developed by the time they are 18-21 years old. Keep in mind that some guys develop early and others develop a bit later. Also, the size of your penis is determined by your genes–similar to the way we inherit certain eye color or skin tone. Beware of products that claim to change your penis size. These don’t work and can have dangerous side effects. If you are having serious worries about the size of your penis, talk to your health care provider. Here’s a tip on how to start a conversation with your provider: “What’s the normal size penis for someone my age?”