Hm DS is 18 months now and it's been going on for probably a month or two, so around 16-17 months? We get it all the time - "waddat? waddat?". Now that he knows what a lot of the basic things are (car, tree, shoe...) he has moved into more detail - now when he sees a car he wants to know what the indicator/keyhole/dashboard etc etc is!
My friend's little girl started pointing at things and wanting to know what they were REALLY early, before 12 months (though she didn't ask out loud); but she is very ahead on communication, they do a lot of baby signing and she knows loads of signs already.