how long babies/kids are breast-fed is primarily a cultural thing - that beeing said, and coming from Europe, I also think that breastfeeding such old kids very strange... (my son, almost 2, is still nursing
with regards to nutrition in BM
it always has some nutritional value - but since the iron-point was mentioned: there are 2 things to be taken into account when talking about iron:
a) the "right level" was set by formula-fed babies, not by BF-babies.
b) breast-milk does in fact contain less iron (compared to formula) but this is not a problem (normally) as the bio-availability in BM is so much higher, so BF babies should get exactly as much iron as needed (although the overall blood-level might be lower - see a) )
I once saw a documentary about the "formula-mafia" in 3rd world countries and it was shocking. key points (out of my memory) were
1) mus were widely mis-informed by dr/nurses as health professionals get money and other benefits if they introduce formula at an early stage
2) if baby weans early mum gets pregnant sooner
3) families ruin themselves by buying horribly expensive formula (thinking it is the best for their babies) and the more kids they have the more money they need to spend on formula, a financial burden a lot of families can't take.
4) therefore, a lot of families water-down formula to "increase the value" therefore making it no value to the infant, causing malnutrition in infants
5) water/bottles are not always clean, again causing a hell lot of helath problems for the babies too
and: mum's diet normally does not affect the quality of the milk within the first 6-12 month as the body takes everything to keep the kids alive (i.e. producing good milk).
just my 2 cents