Shrinkage Is Real

From WebMD's article Life Cycle Of A Penis:

Recent research confirms what men have long suspected and, in some cases, feared. The penis itself undergoes significant changes as a man moves from his sexual prime -- around age 30 for most guys -- into middle age and on to older age. The penis tends to undergo an actual (and irreversible) reduction in size. The reduction -- in both length and thickness -- typically isn't dramatic but may be noticeable. "If a man's erect penis is 6 inches long when he is in his 30s, it might be 5 or 5-and-a-half inches when he reaches his 60s or 70s," Goldstein says. What causes the penis to shrink? At least two mechanisms are involved. One is the slow deposit of fatty substances (plaques) inside tiny arteries in the penis, which impairs blood flow to the organ. This process, known as atherosclerosis, is the same one that contributes to blockages inside the coronary arteries -- a leading cause of heart attack.

Testicles also tend to shrink by one-third by age 60. (Via Queerty)