I think Heaven's post gives a good rundown on the reasons 6 months is recommended. It's based on a lot of research. No doubt there are individual variations between children, but on average digestive systems will be mature enough by around 6 months. And we know breastmilk provides all babies need to this age so it's probably not unti around this age that they need other food.
I think we all agree that it's up to parents to decide, but it's always helpful to have information when making decisions. Jellyfish also asks a very good question - is there any harm in waiting? I like to let my kids decide when they're ready. I am particularly motivated by my desire to delay disgusting solids-poos for as long as possible