We don’t know. Common practice has been to delay introduction as long as possible, but new research out of Australia suggests this practice is dead wrong, at least pertaining to eggs:
From The Australian: New egg and baby study scrambles old thinking on allergies
Over 2,500 infants were tested with egg allergic reaction skin tests starting at age 4 to 6 months and subsequently at 12 months. The study was intended to reveal whether sensitivity at 12 months was related to how long the babies were breastfed, when solids were introduced into their diets, and when egg was introduced. The authors concluded that the initial age at which egg was introduced was significant and the other two variables were not. The later the age, the more likely an allergic reaction.
The original study, “Can early introduction of egg prevent egg allergy in infants? A population-based study,” is gated, published at The Journal of Allergy and Clinical Immunology, Volume 126, Issue 4 , Pages 807-813, October 2010. That copy is gated, costing $31.50. I have not located an ungated version.
The abstract is here.
My biggest question with this finding is how long does the sensitivity persist beyond 12 months? Would infants who just had egg introduced at 11-1/2 months still have a higher likelihood of sensitivity at say, 24 months or 36?