England - the country where your teenage sister and grannie gives birth at the same time!

I had a very interesting conversation during our Freshers Dinner last night, one of many I have to say, with the wife of one of my lecturers. I believe it started with a comment from my part (there was free wine, details are obviously either a bit fuzzy around the edges or simply gone) about all these pregnant young girls in Britain. We might have been talking about Chatham which sounds rather believable to be fair. But anyway...

She told me that the average age in the UK to give birth to your first child is 29. I was a bit surprised. As an ignorant foreigner I have always seen England as the country where teen girls have babies and that is a very common understanding, at least in Sweden. What about all these 17 yearolds pushing baby buggies at the local ASDA? Are they just a figure of my imagination?
After a bit of research I discovered that women giving birth to their first child in the UK are the second oldest in the world, only triumphed by the Kiwi girls. Even the Swedes are behind barely at 28. The UK are 12th in the world in teenage births, however the only western countries beating them are the US and, very oddly, New Zealand.

This got me thinking. How is it possible to have pretty much both the oldest women and the youngest women giving birth to their first child in the western world? Does that mean that there are also grandmas all over England giving birth to their first child? Because I don't know how the maths would work out otherwise.

Stats taken from Nationmaster.com