The latest instalment of our weekly series, in which we use the Centre for Cities’ data tools to crunch some of the numbers on Britain’s cities.

Bloody kids. They’re everywhere, aren’t they? With their fun and their games and their boundless sense of wonder about the magic of the world, the smug little-

Actually, that’s not quite true: as it turns out, some cities are far more child-friendly than others. Data from the 2011 census shows that, in Bournemouth, around 21.1 per cent of the population was aged under 20. In Bradford, at the same time, it was 28.8 per cent – a good third higher.

Here’s the map (sadly, the data only covers England and Wales). As ever, darker dots mean higher numbers, and you can hover over a city to get the details. (Note, May 2018: This data seems to have been deleted from the database. Bum. But here is something comparable.)

The cities with the fewest young people correlate quite closely with those with the greatest number of elderly ones (which we looked at last week): a bunch of coastal towns plus, er, Norwich.

This is probably a matter of mathematical inevitability. By definition, a higher proportion of over 65s means a lower proportion of under 65s. Old people are also less likely to have resident children than people in their 30s or 40s.

The cities at the other end of the spectrum take more explaining. Here are the 10 with the highest proportion of under 20s:

Normally at this point, we’d note that the economies of all these cities have something obvious in common, smugly pat ourselves on the back, and move on. But we can’t do that here: this looks like a fairly random assortment of places. Some of them (Milton Keynes, Peterborough) are boom towns, that can attract young families through both good jobs and affordable housing. But others (Bradford, Blackburn) really don’t fit this profile. So what’s going on?

One thing a lot of these places do have in common is a large ethnic minority population: Peterborough is a centre of Britain’s Polish community; Bradford has a large Asian community; Leicester and Birmingham are two of the most diverse cities in Britain.

This probably isn’t a coincidence. Ethnic minority populations tend to have a younger age profile: because Britain’s diversity is a function of the last 50 years, they tend to have a smaller proportion of elderly people, and a larger proportion of everyone else.

That means that, when you’re looking at the numbers as a proportion of the overall population, you’ll both more young families, and more kids.

That might change over time, though. Check out this graph from the Office of National Statistics, based on 2001 census data:

All ethnic minority populations were “younger” than the white ones. But the black Caribbean one, which dates back to the late 1940s, had a much larger share of elderly people than the Asian ones, which date from a generation later. It seems likely that, as time goes on, the age profile of ethnic minority communities will look more and more like that of the white British population.

But they’ll never get there completely, purely because Britain’s immigrant population tends to be younger too. According to a briefing from the Migration Observatory at Oxford published last year, nearly 38 per cent of foreign-born workers in Britain were in the 25-35 year old age group in 2013; only 25 per cent of UK-born workers were in that group. Because people move to Britain to work, rather than to retire, that’s not likely to change.

One last chart. Here’s the correlation between the proportion of city populations under 20, and the proportion born overseas. 

The correlation isn’t perfect. A few places (London, Oxford, Cambridge) have diverse populations but relatively few kids. (We’re guessing this is a combination of high house prices and large student populations at work.) 

Generally speaking, though, a more diverse population seems to mean more kids.