Infant mortality rates were quite high in most historic populations and there are numerous reasons for this, the one you have pointed out, weaning, has been proven through chemical and palaeopathological analysis of skeletal remains. At this point, I would direct you to a couple of articles that answer many of your questions, and also provide further titles for study; ‘Causes of infant mortality in the Middle Ages revealed by chemical and palaeopathological analysis of skeletal remains’ by G. Hühne-Osterloh and G. Grupe, published in the Zeitschrift für Morphologie und Anthropologie, and 'Infant Feeding Practices inEurope and the Mediterranean from Prehistory to the Middle Ages: A Comparison between the Historical Sources and Bioarchaeology’ by F. Fulminante from Childhood in the Past An International Journal.

One of the research questions of the first article by G. Hühne-Osterloh and G. Grupe, was specifically whether weaning accounted for infant morbidity in the medieval period. By examining skeletal remains, they were able to determine the typical weaning age due to diet changes through the chemical analysis of the bones, and also show that this change in diet often lead to “insufficient or sub-adequate nutritional intake” leading to malnutrition and death. They determined that the weaning age generally began around the six-month mark, at which point the nutritional changes could be observed. Analysis of the bones indicated the nutritional change from a diet largely or even exclusively milk, with the introduction of what they identified as vegetal foodstuff (cereals, vegetables and fruits). Hühne-Osterloh and Grupe concluded that there was both an awareness of this critical period in an infant’s life, and a lack of knowledge of medical treatment of sick children thus indicating the survival of so many children proved intense parental investment.

The second article by F. Fulminante, considers both historical accounts and the use of isotopes to determine breastfeeding and weaning practices from a number of cultures and time periods. The author shows that the time of weaning varied between cultures and periods. Although there was general agreement that weaning began around six months of age, the cessation of breastfeeding ranged from two to four years of age. The author identified a distinct historical trend, a correlation between earlier cessation of breastfeeding and shorter duration of weaning time with increasing urbanization. Meaning that the by the time of the Middle Ages, children were fully weaned at a younger age than in earlier periods.

I think that addresses most of your questions. To sum up, yes, it seems they did start at six months, the solids introduced were vegetal, and weaning was recognized as a risk and required a great deal of parental involvement through the transition period which was marked by a drop of nutrition. What was not presented was how the introduction of solids was accomplished, or which societies were more or less dangerous at this juncture of life, as they all seemed to show similar experiences with infant mortality.