@hawkeye10,
I believe the age 4 thing is an urban myth. I think the average age would be closer to age 2, and in north america it's about 6 months. And that only takes into account those women that do breastfeed, many don't or can't.
I had an aunt who volunteered for La leche league, some of these women are almost militant.. I doubt even they'd agree to that world wide stat.
Regardless, you can't breastfeed when you're pregnant and in some cases, (and like the rhythm method), breast feeding does provide some protection against another pregnancy. Of course, it's a totally unreliable method. In most developing countries people certainly aren't waiting 5 years between children. I'm pretty sure the statistics will prove that.