I'm just reading "Mothering Your Nursing Toddler" which has some interesting stats in it WRT nursing age.
Most mammals nurse until their young triple their birth weight - around a year, in human terms. Which is interesting.
For larger mammals - horses, elephants etc, weaning tends to happen when the young quadruple their birth weight. Around 18 months to 2 years, in human terms. Again, interesting.
However, with monkeys, specifically apes (ie our closest relations in the animal kingdom) - it's the other way around, when their young reach roughly a quarter of their adult weight. Which is anywhere from age 4-6 in human terms. Funnily enough, just when the majority of children choose to self-wean in all studies/counts which have been done.
OK you could say we're not animals and there's no need to behave like them - but we're still mammals and if it's shown to have a biological advantage, how could it possibly be harmful to do this? Nobody's saying everyone has to do it. Just get off the backs of people who do - it's up to them, surely?
As for the argument that breasts are sexual - well kissing can be sexual, but you still kiss your children. It's a totally different thing. Exactly like the difference between kissing your children, and kissing your husband.