I think the more important question (which will give you the same answer) should be, "when should I start to teach my baby to read?".
We recommend 6 months as a good starting point, though many start as early as 3-4 months. Please note that babies are all different and they may respond differently at different ages. Some (probably most) respond the best under 12 months, but some only pick up an interest between 1.5 to 2 years.
Also, to add to the mix, children respond to different methods differently at different ages too! So the long and short of it is that you should just try it and see!
If you haven't already done so, I recommend you download and read the free ebook on teaching your child to read:
http://www.brillkids.com/ext/e-book.php