Thanks VS (must have skipped that lecture at uni )- can't find anywhere that says the process of maturation is complete between 17 & 26 weeks - most places say that it is unknown when maturation is fully complete - it is known that preterm babies have particularly permeable guts, but this seems to be less of a problem once they are post-term.
The WHO systematic review on which the DOH recommendations to introduce solids at 26 weeks are squarely based does not mention anything to do with gut maturation or permeability; the purpose of the review was to look at the evidence to ensure there were no downsides to advising 6 months exclusive breast-feeding.
One study from Belarusse showed a significant decrease in gastrointestinal infections in exclusively breastfed infants compared with those exclusively breastfed until 3 months. This may be for a variety of reasons, not least that formula feeding and giving solid food requires clean water, sterilising of equipment and good food hygiene, in addition to the subsequent lack of transfer of maternal antibodies. Gaastroenteritis is not caused by a 'leaky / immature gut', but can lead to a more permeable gut.
As far as allergies etc, to quote from the WHO systematic review with respect to 6 months exclusive breastfeeding: 'The large Belarussian study (Kramer 2000) found a significant reduction in risk of one or more episodes of gastrointestinal infection. No other significant reduction in infectious morbidity has been demonstrated, and the combined data from Finland, Australia, and Belarus do not suggest a protective effect against short- or longterm atopic outcomes.'
So while there is a hypothesis that allergies may be caused by proteins entering the more permeable immature gut (particularly cows milk protein), it remains a hypothesis.
And I stand by my point that breast milk is good stuff, it's got better things in it than formula, and the longer it's given, the better.