Simone C that is absolute rubbish. EVERY paediatric board recommends bfding for at least a year and most recommend 2.
Breastmilk does not change in constittion in any way - it still has antibodies, enzymes, live cultures etc.. Although complimentary food is important to supplement for things like iron, (some studies say that prolonged breastfeeding is associated with nutritional inadequancies, but this is only if complemantary foods are not introduced correctly) milk feeds are most important for any baby untill the age of one (the reason you have to stick with those bottles if you are formula feeding)
A childs immune system is only fully mature at about 5 years old, so breastmilk supports an immature immune system.
Other great apes feed their young till immune system maturity (Chimps for 4 years) and anthropologists believe that the natural weaning age of humans is that as well (which is why most children have an urge to suck on a dummy etc for that long)
Prolonged bfding helps reduce certain cancers in both the mother and the baby, and there is some research going into the use of human milk in fighting cancer in ADULTS. It shows a significant reduction in the amount of childhood lukemias. Reduces a moms risk of breast and cervical cancer (bfding for 1 year reduces your risk by up to 30%, another year by an additional 15%,etc..)
I'm not saying that we should all bf for years, just the myth that it looses its value is one that has been perpetuated by the modern world who undervalues nature and likes to rely on boxes of processed, genetically modified foods and supplements in tins..