At some point, as the Greek crisis lurches to its crescendo, Syriza – the radical left party – will call a meeting of something called a central committee. The term sounds quaint to 21st-century ears: the committee is so big that it has to meet in a cinema. You will not be surprised to learn that the predominant hair colour is grey.

These are people who were underground activists in a military dictatorship; some served jail time, and in 1973 many were among the students who defied tanks and destroyed a junta. But they think, speak and act in a way shaped by the hierarchies and power concepts of 50 years ago.

The contrast with the left’s mass support base, and membership, is stark. In the average Greek riot, you are surrounded by concert pianists, interior designers, web developers, waitresses and actors in experimental theatre. It is usually 50:50 male and female, and drawn from a demographic as handy with a smartphone as the older generation are with Lenin’s selected works.

Like young radicals across Europe and the US, they have been schooled in the ways of the modern middle classes: launching startup businesses, working two or three casual jobs; entrepreneurship, loose living and wild partying are the default way of life. Of course, every generation of radicals looks different from the last one, but the economic and behavioural contrasts that are obvious in Greece are also present in most other countries.

And this prompts the question: what do the radicals of this generation want when they win power? The success of Syriza, of Podemos in Spain and even the flood of radicalised young people into the SNP in Scotland makes this no longer an idle question. The most obvious change is that, for the rising generation, identity has replaced ideology. I don’t just mean as in “identity politics”. There is a deeper process going on, whereby a credible identity – a life lived according to a believed truth – has become a more significant badge in politics than a coherent set of ideas.

The most important picture of Ada Colau, the leftwinger who won the mayoral race in Barcelona, shows her in a green T-shirt being arrested for resisting bailiffs. The most important rite of passage for a radical public thinker is summarised in the acronym AMA – the “ask me anything” sessions pioneered on Reddit. It is a realtime test of sincerity.

Conversely, young people are not so interested in fixed political programmes. There has been no significant outcry among the young Greeks who voted for Alexis Tsipras as he has watered down his party’s original programme three times over. One told me: “If they only do one thing they promised, I will still vote for them: for ever.”

The implication is: if political parties deliver against a core belief, with integrity, and do not act like perennial crooks, then no matter how minimal the change they achieve, it is the integrity that matters.

The Russian revolutionary Trotsky once instructed his followers: “Programme first, your political passports please!” Today your political passport is your face, and its tendency to look shifty when lying. It has to be presented in selfies, vox pops or the high-definition cameras on live TV. The problem comes when you try to translate the politics of sincerity into action in the official world, where accountability is measured against budgets, manifesto pledges and party political deals. The new parties of the populist left will tell that you that it’s not easy.

As I’ve covered the rise of Syriza and Podemos, including the municipal battles in which they cut their teeth, it has become obvious that, for many in this new generation, winning political power is not an end in itself. Neil Kinnock once called the Labour party a “dented shield” for the British working class to protect itself with: not pretty, and not a sword, but functional. More than 20 years on, the phrase describes the way young people are using political parties – to do the “ruling” bit of a far wider project.

For the present, radical left parties ruling towns or small countries have tended to rest content with doing one big thing: usually defending existing public sector institutions. The really interesting phase comes when they begin to design new things. Because the values, lifestyles and tendency to networked mercuriality of the new generation means that, if they started from scratch, they would not build the same things their grandfathers did.

A 25-year-old who has known only temporary work, on an individual, performance-driven salary, is going to find the concept of a wage agreement puzzling. A young person who expects to move jobs, cities and even countries several times over in their working life would, if asked to design a welfare state from scratch, do so in a way that allows people to carry their entitlements with them. And designed from scratch today, by a generation hooked on choice and networks, it is unlikely the NHS or education would come out as monolithic single systems.

The strange thing about the move to radical populism in politics is how much, so far, the young have deferred to the systems designed by the old. Syriza, for example, has dedicated a whole subsection of the Greek cabinet office to fostering “peer-to-peer” and the sharing economy. But it is not exactly prioritised in the narrative.

I think we will know that a real new left has emerged when we begin to see its thinkers prioritise the redesign of institutions inherited from the 20th century, and the invention of new ones centred on the self, identity and structured to survive incessant change.