What these doctors say is true. I have seen it. At the age of 40 my penis began to shrink. I did not know this happens to everyone. Yes, your penis gets smaller as you get older.
When younger, I could not wear boxers or 'go commando' as they call it, because my penis would constantly slap against my thighs all day long. But now, it does not even reach my thigh when flaccid. *shrinkage*
Every decade since 40 my penis shrinks about an inch. Happens to everyone. Currently, I am around about two inches and the shrinkage has not stopped. Eventually, I expect that my penis will simply disappear or it will become not noticeable if I live that long. It's all part of the process of getting old.
Anyone under 40 should enjoy the days while they last! Eventually, your penis will be long gone and forgotten.