Babys have what is called an extrusion reflex which if food is put in the mouth its spit back up, which makes it seem that baby does not like :( BUT that is not the case. However, until this reflex diminishes, which is at the average age of 6months, then cereal cannot be given.
Now "average" is the big word here. Ped's say 4-6 because some children may develop faster than others. usually 6 months is the mark though.
I tried at 4 months and he pushed it out... tried at 5... and still the same response but not as strong.... by the middle of 5-6months... I tried again and PERFECTO!!!
So alot of it isn't necessarily allergies to cereal but more because they just can't get it down yet. ALSO if you are breastfeeding the 6 month mark is vital. If you start to introduce cereal earlier than that means less breastfeed times.
Hope this helps, good luck.