In preparing for an. upcoming conference in Manchester, UK, I noticed this odd-looking population pyramid on the Wikipedia page (CC-BY-SA by Wikipedia user Tweedle).
I thought it looked like a population Christmas tree. What accounts for the sharp transition between 15 and 20? This is a city of over 500,000 people, so it must be something real. My main theories were: data/chart error, counting artifact affecting minors, and immigration age restrictions.
First I had to get the data the verify it for myself. Nicely, Tweedle provided a link to the UK census data broken down by sex and age. I looked at many UK cities and most had one of these three patterns. (never mind the inadvertent color swap)
Still confused, I posted to my socials (which for me these days are Mastodon and Bluesky). A few people on Bluesky responded that it was the universities. It didn’t occur to me that there would be so many university students, but it was right there in the Wikipedia article: 76,000 students in Manchester.
Apparently, Manchester has the pattern of students coming for school and mostly staying to work. In cities like Canterbury (also smaller), students come for school and then leave. The shape of Swindon mirrors the population pyramid shape for the UK as a whole, with the dip around age 20 being part of a cycle.