Ben started to wave bye-bye since few weeks ago (7 to 8 months old). Each time when daddy is about to go out for work, daddy will wave and say bye-bye to him and he will wave back.

Poll result from babycenter.com shows this:-

How old was your baby when he waved for the first time?
5% Younger than five months
10% Five to six months
45% Seven to eight months
32% Nine to ten months
9% Older than ten months
(Total votes: 61690)