When we asked our international expert advisers what age they thought characterised a "young" university, the responses ranged from 100 years to just 20.

In a world where some of the most cutting-edge research and teaching takes place in medieval buildings (but where much younger institutions, with the right support and governance, can also be world class), it was never going to be easy to reach a consensus: in the UK, for example, the universities incorporated in 1992 are still regularly referred to as "new universities" 20 years on.

But the majority of the Times ÖйúAƬ-Thomson Reuters' "platform group" of more than 50 experts from 15 countries homed in on 50 years as a landmark age. Five decades seemed to offer enough time to develop bodies that could compete with the best research-led universities, typified in the UK by the "plate glass" institutions established in the 1960s, such as the universities of Warwick and East Anglia.

The 1960s constituted a definitive era in ÖйúAƬ, particularly in the UK. The economist Lionel Robbins was commissioned in February 1961 "to review the pattern of full-time ÖйúAƬ in Great Britain and...to advise Her Majesty's Government on what principles its long-term development should be based".

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His 1963 report identified "the existence of large reservoirs of untapped ability in the population" and heralded the "Robbins principle" - that university places should be available to all who were qualified for them by ability and attainment.

Although some of the 1960s institutions most associated with the "Robbins expansion" were already open for business by the time the report was published, the document set the tone for mass expansion.

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Foundation dates

Reaching a consensus that 50 years is an appropriate time period upon which to base the analysis was one thing; establishing a firm foundation date for the institutions ranked was another. After all, even some of the newest institutions can trace their origins to the distant past.

In gathering the data, Thomson Reuters' basic principle has been to take the date on which the institution was founded if purpose-built as a university, or the year it attained degree-awarding powers if it evolved from another type of educational institution (see box below for the full criteria).

Of course, in several cases we have had to use a significant amount of editorial judgement, and we are open to those judgements being challenged.

For example, although France's Universite Pierre et Marie Curie can trace its roots back to 1109 and the old University of Paris, Thomson Reuters took local advice before agreeing to accept 1971 as its foundation date, given that it was created as a substantially new institution at that time.

However, we expect and encourage dialogue with individual institutions about their origins: please contact rankings@timeshighereducation.co.uk with your thoughts.

The 50-year rule

It should be acknowledged that there is an element of cruelty in our choice of half a century, which means that 1962 is our strict cut-off date. This is tough on some: it means, for example, that we include the UK's Keele University, founded in 1962, but not the University of Sussex, another strong performer, which was founded in 1961.

Such cruelty will continue. The 100 Under 50 list is not a definitive ranking of record like the THE World University Rankings: it is designed to be a fluid and dynamic list. So next year's table will stick with the 50-year rule, cutting out all those institutions founded in 1962 and before. The year after, those founded in 1963 and before will be excluded, and so on. As such, a great degree of change is possible from year to year.

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We think this is justified in a forward-looking analysis that seeks to examine potential and emerging forces, not to establish a definitive list of the world's best institutions irrespective of age and history.

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As Rabindranath Tagore, winner of the Nobel Prize for Literature in 1913, said: "Age considers; youth ventures."