Interesting thing to consider. As a child we had a fish tank and in it, a Siamese Fighting Fish. If you held up a mirror to the tank, it would flare it's fins and charge at it. They're very aggressive towards fellow males. In fact, if you were to put a mirror in the tank it would likely die from exhaustion, due to a never ending garage of attacks. I have a one year old and we spend a lot of time these days waiving "hello" and "goodbye" at his reflection. I look forward to when he realizes he is waiving to himself.
According to this, around 20 months or so is when babies realise "that's me" http://www.spring.org.uk/2008/05/when-self-emerges-is-that-me-in-mirror.php
It's interesting how a child passes through a stage of withdrawal between "another baby!" and "it's me!" My parents have told me that I was terrified of the full-length mirror in their bedroom for quite a long time, so this self-conscious withdrawal makes sense.