My little one is a lady of four now. And its been quite some time when she started asking about 'how' mamma papa got her. Of course this had to happen. Some kids ask about it later, some ask earlier. My baby started asking me about the same when she was about three-n-half.
Earlier, Indian parents were more secretive about most topics that skirted around issues of a physical and sexual nature. Such talks were almost a taboo, not almost, I think I can say to a certain extent, they WERE taboo. In fact, most Indian households still are not comfortable talking to their children about issues that involve anything to do with sex or the human body.