In most men, testosterone levels and natural production peaks at somewhere between 18 and 25 years old. After that, it is normal in a great percentage of men for testosterone levels to fall. I have yelled and screamed that every man when he is between 18 and 25 years old needs to have both serum and free-floating testosterone levels as part of their permanent medical record. The Food and Drug Administration of the United States Government is the organization that dictates what treatments an MD can offer inside U.S. borders. There are two different guidelines used in U.S. Medicine regarding male hormone replacement therapy. The first is that testosterone must fall dangerously low and unless it is below 200-300 count on the serum level you do not get replacement. There is however an exception. If your MD has as part of your medical record that your testosterone level was normal and documented at 600-800 and it has fallen to 400 you then qualify for replacement. There is another exception. This exception regards the conversion of testosterone into DHT. If your body ceases to be able to convert, you again qualify for legal and medically necessary testosterone replacement. At that point you receive either injections or DHT trans-dermal cream to raise that level.
Most testosterone esters are recognized by the pituitary gland and as a result the pituitary down-regulates production. Medically this balance is referred to the "pituitary axis". There are times that in males the problem in lack of testosterone production in the first place is not the testes but is instead lack of pituitary stimulation. It normally takes an Endocrinologist specializing in this area to be able to sort out an exact cause.
The only testosterone ester not recognized easily by the pituitary is DHT itself. Dosages of DHT have to be very high before down-regulation takes place.
All people are different. Some men given supplemental testosterone will down-regulate quickly and will show testicular atrophy quickly, while others will not down-regulate quickly and in these individuals that same atrophy will take a very long time.
Medically, there is a preventative for testicular atrophy and this is a series of injections of a stimulating hormone referred to as Beta Human Chorionic Gonadotropin. This is again brought into play to kick the testicles back into "warp-drive" and increase production.
This sounds simplistic and in fact is over-simplified. Recreational use of hormonal compounds has potential to do long term harm and in many ways not normally discussed. Many men for example when placed on testosterone cypionate (Depo-Testosterone) have major alterations in red blood cell production which thickens the blood and in a few has created catastrophic problems.
I grew up in a medical family and for numerous years of my life was surrounded by numerous medical doctors as part of a very large social circle. I was "groomed" by a Surgeon Step-Father to head into medicine myself. Being the rebellious kid I was I ended up with a degree in music. The quest for knowledge in this area has never ended and as far as I know, the information I have is still current and accurate as of 2007.
For the absolute truth regarding the usage of testosterone and it's effects the best places to do your own research are with studies relating to the higher dosages given to HIV patients dealing with the wasting syndrome.
It has been said more times that we can count in this thread and many others!
Unless there are some unusual circumstances and an endocrinological abnormality to begin with, no form of supplemental testosterone will increase the size of an adult male penis.