This is a long post, apologies, but it had to be because this is a complicated subject and reducing it to soundbites is a travesty.
I am really, really uncomfortable with the sweeping statements on this thread.
Early life nutrition is clearly very complex and so is allergy, and the risks of feeding one way or another are hard to research and poorly understood.
In the UK where WHO advice is government policy, the percentage of children who follow the guidelines and are exclusively breastfed to 6 months or later is around 1%. The rise in allergies has been happening for a couple of decades.
The huge rise in allergy has been extensively investigated as you would imagine and it still largely remains a puzzle. But you can't attribute it to something that only a tiny percentage of the population is doing.
As far as the evidence goes - and it's very scanty - there's reasonably strong evidence that allergy risk is increased if you wean before about 3-4 months. Weaning age doesn't seem to have an effect on allergy risk after that. (American Academy of Pediatrics review).
The question of which age is best to introduce a baby to particular proteins in order to minimise the risk of allergy is very interesting, but nothing is known for sure, except that breastfeeding at the time of introducing the food appears to have a protective effect. All the rest is speculation.
The opinion quoted below, that the recommended weaning age will be reduced to 4 months, is very far from mainstream. (Doesn't make it wrong - but most people in the field believe the evidence points differently).
What is believed to be far more likely as far as I can see from what I've read, is that ideal weaning age will turn out to be a very individual thing, a balancing act for each child between different kinds of risk.
Sadly for those of us with little childen, we're light years away from being able to balance the risks for an individual child. Population data is all we have, and it's imperfect. But it's better than nothing, and "nothing" is what our parents had. We should be grateful that we at least have a few pointers.
Why is all this important? There's very strong evidence from various sources that early life nutrition has a "programming" effect on metabolism, and this can have a lifelong effect on risk of various conditions such as heart disease.
So, frustratingly, all we know is that what we feed our children could be terribly important - yet we don't know beyond a few basic facts, how we should be feeding our children.
However, one thing is very clear from thousands of studies, and more are appearing every month: whether a baby is breastfed or not, and for how long, can potentially have a profound effect on their health through life.
Stopping breastfeeding should never be a casual decision - it's possibly the most important health-related decision you'll ever make for your child.