Not a medical opinion just a personal one, but I see it that nothing magical happens on their birthday, kwim? I guess my own way would be to go a bit longer rather than a bit earlier, but depending on what situation I was in, I wouldn't see a big deal with a couple of weeks. As I said we were told to stay on formula and not to a milk-challenge until 18 months, I did it at about 14 months or so just because of our situation (label reading in a foreign language was a total PITA!) and my DS was fine...I think it is just balancing out the info you have and choosing, if that makes any sense!