Most people start to figure things out by the end of their third year if they haven't already. Or, they do a couple of end-of-third-year/early-fourth-year electives to narrow down their final decision. If you know early on, great...you can get involved in area specific activities that show interest. But, most programs understand that people may not know what they want to do until they do a particular rotation. Most of the people I know ended up changing their minds from what they originally considered at the beginning of med school. Heck some people switch between fields as residents; although, this isn't the most envied time to make a switch, but it's better than being stuck in a field that you turned out not to like.
Just keep an open mind about the various fields, and you likely won't go wrong.
