How Old is the Earth ?

I am by no means an expert on radioactive dating. But I heard for many people (mainly creationists, of which I am one) that are flaws in this method.I heard that radioactive dating relies on too many assumptions. One assumption was about the amount of carbon or other material that was originally present in the material dated. On the same note, the amount of the daughter material is also assumed. Examples of incorrect radioactive dates were given through the dating of materials whose dates were previously known. For example, a 200-year-old lava flow was dated to be around 3 billion years old.It sounded like a great argument, and I was wondering if anyone had heard anything about it. Has it been disproved, supported, etc? Please respond.