From YourTango

Is he acting crabby again? Turns out, the aging process might be to blame.

Introducing "grumpy old man complex" (also known as "irritable male syndrome"). The cause? A drop in testosterone that accompanies aging. "Testosterone is a hormone that grows muscles, reduces fat in the body, affects energy and improves sexual desire," Dr. Ridwan Shabsigh, New York City urologist and head of the International Society of Men's Health, told MSNBC. "However, it also has neural-psycho effects. And in some men we encounter in our practice, those affects can be mostly visible: low mood and irritability."

This grouchy outlook typically hits around age 60—so technically there's no reason for him to be yelling at the neighbor's dog until then.