Why wait until six months to wean?

The argument for waiting until your child is six months old
The World Health Organisation - who monitor the health of people from every part of the world who live in a variety of good and bad conditions - now believe that breastfeeding is so important that it should be the only source of nourishment for a baby until he is six months old. They state that their policy is that a baby should be given no other liquid (including water) or solid food until six months. They also believe breastfeeding should continue until the child is two years old.
This is due to worldwide concerns about infants getting adequate nutrition. Once the move across to solids begins, it is common for the balance of nutrition to go wrong, where breastmilk offers a range of nutrients. Their stance is very focused and does not allow for the inability to breastfeed exclusively - due to mother's medical needs or because a mother is returning to work or has to be away from the child for certain periods of time, for example. However, as a world-view it is understandable.
The greater concern from health bodies in the UK is that giving babies solids much earlier than six months can lead to obesity (according to recent research) or contribute to the development of allergies.
Those parents who know they have a history of allergies in their family will be advised accordingly and probably encouraged to use solids sparingly between six and eight months and to avoid certain key 'allergy trigger' foods for much longer.
There is also a concern that any baby's digestive system is not mature enough until about six months and that having solids before then can harm organs like the kidneys. Weaning any earlier than four months is certainly not advised in any situation.

Why it's important to start feeding solids at six months
Breast might be 'best', but there are key nutrients which a baby needs by the time he is six months old (it lacks iron, for example). Breastmilk alone will also fail to provide adequate calories for your child.
Once your baby is sitting up, it won't be long before he is moving around and he will need more and more calories to grow. If he wakes in the night because he is hungry, this sleep-disruption habit will be harder to break the older he is.
Although formula milk does have added nutrients by this stage, it is important for your growing child to become accustomed to eating food and developing his eating abilities. It is very likely that he will not be eating confidently or very much for a good month or so, and to delay it for too long only prolongs the period before he is taking in the right amount of solids. (The World Health Organisation recommend feeding solids two or three times a day between six and eight months. However, a feed can just mean one ice-cube sized portion of simple, pureed vegetable.)
As you will discover over the coming years, new babies are far more open to new ideas! Exposing your baby to different tastes now - not neccessarily exotic ones but certainly a range of different vegetables or fruits - helps keep your options open for meals in the years ahead.

What if your baby is hungry before six months?
There are a couple of main reasons why parents might want to start feeding solids earlier than six months.
Firstly, your baby might show signs of getting more hungry. This doesn't happen with all babies, but signs include: waking in the night after he had got into a sleeping-through routine; showing an interest in your food at mealtimes (even though this doesn't mean you should give him any of it!); he wants more milk feeds or is still hungry at the end of a feed. Secondly, you might be returning to work or your child is starting to attend nursery sometime between four and seven months. Getting a child onto their first solid foods is a laborious task and requires patience, a good focus, time and the right frame of mind from baby and parent. If the household regime is changing at six months with the noticeable absence of mummy, it would be a good idea to try to bring the major eating shift forward a little so that these two things don't collide. Additionally, few nursery staff will have the time, even if they have the inclination, to get your child onto solids for you.
If you keep your baby's diet simple but nutritious, you should not feel too worried that they are going to suffer too badly from starting solids at five months. Keep yourself informed about essential baby nutrition and feed sparingly so they are used to the action of feeding without taking in so much quantity that their system is challenged or that they stop feeding from the breast as much as they should be doing at five or six months.
As mentioned above, if your child has an increased risk of food allergies, talk to your doctor about the transition and don't bank on being able to feed him ahead of the recommended schedule.

Other issues which affect successful weaning
Six months might sound like a neat time to start weaning, but apart from hunger and the changes to your daily routine (like returning to work after maternity leave), there are a couple of other things which collide with this timing.
Firstly, although babies do sometimes sprout their first teeth earlier, six months can be the age when your baby is teething on a major scale. Sore gums, upset tummy, pain and crying are not great starting points for the whole new oral experience of eating. This is not neccessarily a reason to bring forward or put off weaning, but be sensitive to these times and ease off feeding solids on the days when you child seems to be really suffering.
Some teething babies might enjoy the cooling, soft puree but others will find it an annoyance.
Now that your baby is sitting up, he might be more easily distracted. If you do have a child who is already reaching out and almost crawling - this isn't the norm but does happen - then make sure you turn off music, try not to chat to other people yourself and ensure that you see this often slow process as quality time. Some singing or light reassuring chat from you can help smooth out the feeding experience.

If you want to see what the World Health Organisation has to say about breastfeeding and weaning, visit the WHO website.