My son's started about 4 weeks old while I was breastfeeding. It was a physiological problem that had nothing to do with diet or allergies in any way. They did barium x-rays because they thought we were dealing with pyloric stenosis (which would have actually been better because a simply surgery would have corrected the problem), but the reflux showed in the x-rays and it was just what we had to deal with.

All the throwing up did give him a very touchy gag reflex making getting off of pureed foods to anything with even the slightest texture very difficult, but I'm happy to say by the time he was about 18 months, it had all worked out and he was eating with no problem and no more throwing up.