Jan 15, - Touchy subject. The truth is the penis will shrink a little as time goes on as a result of decreased blood flow and testosterone. “By the time a guy is in his 60s and 70s, he may lose a centimeter to a centimeter and a half in length,” explains Dr. Castellanos. You're getting wrinkles. The candles on your birthday cake barely fit. Old age sucks. But it gets even worse. Your penis will begin to shrink – or does.