I was young with no support from x-H, family etc... when I weaned the first one so it was at six months. My second was 20months and my third was 18 months. At around twoish kids are starting to really explore their environment, are eating solids well and there is a natural window for weaning. My babies were ready to move on. Mom is often ready to have her body to herself again too. If you go past two then Mom making the decision to wean gets much harder as baby becomes MUCH more resistent. Then you move on to child led weaning which typically goes on until three, four or five depending on the child. This is my personal view of things and has worked well for my kids. I have been lucky not to have any with milk allergies although to your point cow's milk is for cows and human milk is for humans. It is no surprise that some people have trouble with it.
