However, this is not what happens in most further education colleges, where the age mix is far more diverse. Yet there has been no serious research into whether this is a good thing. We do not know whether students do better academically or socially, nor whether students and staff prefer mixed-age groups.
The question is not just interesting for academic researchers. The 47 local learning and skills councils are carrying out strategic area reviews, to ask for the first time what is the best way of organising FE colleges, school sixth forms and sixth-form colleges.
So how is an LSC to decide how to balance age-mixed and age-segregated institutions? It would be easy if there was clear evidence of student preferences, or of academic performance between different kinds of learning group, but comparisons are extremely difficult.
The pattern of post-16 education varies enormously. In some places a single FE college is the only provider of A-levels, basic education, GCSE resits, vocational courses, evening classes and some kinds of higher education. In others, the college competes with strong school sixth forms and sixth-form colleges.
The benefits of age-segregated learning are well known, and many parents will see a sixth form as the safest environment for their offspring. But does mixed-age learning produce distinct benefits?
Research from the universities of Surrey and Sheffield, funded by the Learning and Skills Research Centre, has shed some light on the question.
The first finding is that the issue is not simple, nor easy to measure. Age mixing happens because of timetabling practicalities and efficient group sizes, and is rarely planned or monitored.
Nor are FE students restricted to either mixed or segregated classes: a typical student moves between different kinds of group across a week, and a class with a few mature students in October may have lost them all by January. So quantitative comparisons of age mixing and academic performance are impossible.
But this does not mean that there is no evidence. On the contrary, the research found overwhelming and consistent evidence. Almost without exception, staff, students and managers interviewed in six colleges in the South-east and Yorkshire believed that students in mixed-age groups behave better, are more motivated and achieve more.
Sixteen to 19-year-olds said that they would have "messed about" without the tone set by the older students. Older students welcomed the liveliness of the younger students and often felt that without a mixed group there would have been no class at all. Teachers found that the influence of other adults made teaching easier. Asked whether they would recommend segregating by age, the response from students and teachers was strongly against.
Of course, this does not mean that many students are not better off in age-segregated institutions. But it does mean that many students prefer age mixing, and that stricter segregation on age grounds would deprive some students of things they value highly.
However, most FE colleges are large, complex and confusing places in which it is easy to get lost among perhaps 10,000 students. Ministers are rightly concerned to ensure that young students have clear points of reference - that someone knows where every student is and what they are doing. This does not necessarily mean separate institutions: it may mean more sophisticated pastoral and tutorial systems, as some colleges have been putting in place.
In developing strategic area reviews, LSCs will need to pay attention not only to the Government's concern about "a distinct learning environment" but also to "respecting learner choice". Ministers have indicated that guidance will be issued soon, but one thing is clear: although many students interviewed had not chosen age mixing, fewer than one in 10 would choose to go back to segregation.
Learning Together: Age Mixing in Further Education Colleges will be published by the Learning and Skills Research Centre in January.Stephen McNair is professor of education at the University of Surrey, and co-director of Learning Together with Professor Gareth Parry of Sheffield university