Teachers are getting older. That is an article of faith that may need rethinking over the next few years. According to these figures from the Organisation for Economic Co-operation and Development, the UK has one of the highest proportions of teachers in their 20s in the developed world.
But what determines the age balance of a country's teaching profession? Partly, it is the length of its higher education system. A degree, and frequently postgraduate training, is necessary to become a teacher in most countries, so entry before the age of 22 is impossible.
Add in the popularity of a gap year and, even in those countries with a three-year degree programme, a new teacher cannot start work before they are 23 or 24.
In countries such as Germany, where higher education takes longer and competition for teacher training places is great, the age of entry may be even higher, meaning there are few teachers in their 20s.
But taken alone, the time spent in higher education would not explain why there are now so few teachers in their 20s in some European countries. One possibility is the changes in working lives of women over the past half century.
In Britain before the Second World War, most women teachers were obliged to stop work when they married; before the equal opportunities legislation, rampant inflation and house-buying boom of the 1970s, women still largely quit work when they started a family.
This meant teaching needed a constant flow of new young women entrants, especially in the primary sector that had been mostly staffed by women. Once women's working lives began to look more like those of their male colleagues, teacher turnover dropped significantly.
Finally, demographics play a part in determining a country's need for teachers: a falling birth rate and the number of young teachers declines as new recruits are no longer needed. Over the next decade, most countries will undergo a re-balancing exercise as large numbers of teachers in their 50s reach retirement age and are replaced by newer, younger entrants.
John Howson is director of Education Data Systems, part of TSL Education.