Ask your pediatrician. Different ones have different views, but ours said there's nothing magical about the 1st birthday, as long as they're close, and they like the taste of whole milk, and it doesn't seem to affect their diapers, go for it. Some babies will have runny diapers for a few days if they're put on whole milk too early or too fast. Ours turned 1 on Dec. 26, and we forgot to bring extra formula to Thanksgiving trips, and I called and asked if he could go to whole milk at 11 mos, and they said he could, but it would best to wait until after the holiday because too many changes at once could affect his tummy as well (different foods, different people & sleeping habits). So some suggest to do it just before the first birthday so they're not having lots of new things all at once (cake, lots of ppl around...etc).