at what age does a man's penis (usually) stop growing? up to what age can it grow?

Answer 1

A male will grow until he is 21 years old. As for the penis, the size is determined by genetics. Medicine will tell you that you need to wait until 2 years after you stop growing to know what size your penis will be as an adult. So, you probably won't know that information until you are 23. For more info:
http://menshealth.about.com/cs/penishealth/f/penis_growth.htm

BTW, the "average" size of the erect member is 6". If you'd like to see for yourself, please refer to this study:
http://wwwu.edu.uni-klu.ac.at/amiklaut/dr_nick/answer19.htm