I would aim for starting at 6 months as I think 6 months of just breastmilk is optimum for my baby, but yes you're right babies do develop at their own rate, and I've heard from quite a reliable source that the NICE guidelines will be changing to reflect this, but that's hearsay and I have no proof to back it up
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As far as I'm aware, most evidence compares weaning before 17 weeks with weaning after 6 months, so while we know before 17 weeks has been shown to increase risk of allergies, and weaning after 6 months to lower risk in comparison, there is little hard evidence in the stage between this. The logical jump of course is that the nearer a child is to 6 months the lower the risk.
I aim(ed) to breastfeed exclusively until six months and unless there was a strong indication otherwise wouldn't expect to wean before then. I was spooning mush into dd1s mouth as close to 16 weeks as possible and BLW with dd2 just after 6 months and would definitely say the latter was easier and more desirable to me.
Individual babies do have small differences in readiness for food just as they have different milestones in other ways which a parent will notice if in tune with their babies needs. Therefore it isn't unreasonable that one baby will be ready developmentally and their system physically ready for solid food slightly earlier wheras another won't be ready until 7/8 months.
A change in the guidelines (which I previously suggested is on the cards) will be a double edged sword I think. It may stop some of the snottiness sometimes seen when a mum genuinely sees the appropriate cues in her child a little earlier than the six month holy grail which to me would be a good thing, otoh though I can see things being harder for those whose children aren't ready until nearer the 7 month+ mark and a return to babies being weaned earlier than they are ready. 'My baby was definitely ready' will become the default for most parents as parents read that it's fine to wean around 4 months again and with that 'there's no harm in doing it a little earlier' will almost certainly mean little ones weaned at such an early stage as to be potentially harmful .
I do think that in a lot of cases weaning is a milestone parents just can't wait to get to and why some parents love the mess and hard work of weaning so early is beyond me.
I think the best thing to do is aim to try weaning around 6 months, but to be in tune with the needs and cues of your individual baby. 'Real' food is rarely more filling or nutritionally of more value than infant milk, so there is rarely a good reason to replace that milk with food at a very young age, and some evidence that it may actually be harmful.