In the millenium cohort of 15,980 children there was no increased risk of hospitalisation with diarrhoea or respiratory infection for children who started solids under six months.  Here.

There are a lot of things we make parents feel bad about with regard to feeding, and the age at which you start solids is, to my mind, one of the more bizarre.  This helpful study can give comfort to parents of children in the UK - perhaps broader developed world - who feel, for very strong reasons, that their child is hungry and should start on feeds under six months.

It’s depressing that most advice about feeding - which internationally is probably reasonable - is instead manipulated in ways which might be less than desirable.  So, you end up with parents buying formulae “for the hungrier child” rather than starting some solids.  You end up with parents keeping their child on follow-on feeds because of ever present guilt about cessation of breast feeding.  You end up fueling the marketing machines of companies which like to identify a problem and then sell you a product to fix it.