I started 2 weeks after my son was 11 months. I did it gradually and by 1 year, he was completely on cow's milk. The transition was much easier than with my daughter, so I recommend this approach. I just mixed a small part regular milk with his formula and gradually increased the amount of milk. I don't even have to heat it up anymore, which is great! Now, if I could just get him to drink it from a hard spout sippy cup, I would be all set!
There is no problem starting your baby a little early on the milk...in fact, my pediatrician was the one that suggested it. I think they want to know how they are able to tolerate it so that you can discuss any problems with them at the 1 year old appointment....makes sense to me! Good luck.