When puberty ends, there is no real or healthy way of increasing a person's penis size. There are no miracle pills or exercises for this and TeenHealthFX highly recommends not using any creams or products that say they will enhance your penis size. For more information on this see TeenHealthFX's response to Want To Enlarge My Penis.
It is possible for the penis to still be growing at age 20. It all depends on when puberty started. However, if you are nearing the end of puberty, the penis may grow slightly in width rather than length.
TeenHealthFX's most frequently asked question from its' male users is "Is My Penis Size Normal?" Please check out TeenHealthFX's responses to these questions to see what is normal and what is not in terms of penis size and testicles: Penis Size, 3-Inch Penis, Can I Make It Bigger?, Average Testes Size, Afraid Penis Is Too Small For Sex and When Does The Penis Grow In Thickness?.
If you have any other questions, please feel free to write back to AskFX or search our website for more information. As always if you feel that something is not right, get it checked out by your doctor. If you do not have a doctor and live in northern New Jersey, please call the Adolescent/Young Adult Center for Health at (973) 971.6475.