I just found a baby Mallard duck...Can you help me determine how old it is?

I was on the river today and there was a baby duck all by itself in the middle of the river. We sat and watched it for awhile, and it came right up to us, crawled onto the bank, and immediately imprinted itself. We tried to leave it without touching it, but it followed us all the way up to the car. Eventually, we just took it home. I've been reading up on how to care for it, but advice would still be good. We're going to keep it in the bathtub for awhile, and we're going to feed it cornmeal until we can get real feed. Can anyone tell me how to tell how old it is? Or if it's a boy or a girl?