Current advice is still wait until around 6 months. As with all health advice, it is based on the most sensible interpretation of the latest research. Esme is right that the cause of allergy and other health conditions are not fully understood, but the research does show that early introduction of solids is more likely to result in allergy later. This does not apply to every child, but who knows which child it will apply to. One of the reasons early weaning leads to this is that the schedule is not followed closely enough - stage 2 foods are introduced when the baby should only be on stage 1 as the carer has run out of ideas may jump to the next range of foods. And as Saffron points out, the bacterial balance of the bowel is altered. There is a huge amount of research in this area linking bowel bacterial imbalance with autoimmune conditions and psychological symptoms. One thing to watch out for is salt before 1 year. In the Children of the 90s study (ALSPAC) of 14,000 mothers and their children, salt levels were worryingly high - main sources being bread and milk, introduced too early to under 1s.
