Egypt, Tunisia, Libya, Bahrain, Syria and Yemen are important reminders that surface calm should not be mistaken for social cohesion and that economic growth – by itself – is often not enough and can even increase inequality. Regimes that had seemed invulnerable can quickly fall.

But might the northern African uprisings spread south? Of course, Africa south of the Sahara is not a single political and economic entity. There are states in conflict, reformers and laggards, small and big countries and those dependent on natural resources. All have their own complexities and challenges.

A group of reformers – including Ghana, Tanzania and Mali, among others – have grown their economies at more than 5% annually since 1996, in the process radically reducing poverty and mortality rates. This has been on the back of improved governance, higher commodity prices and the mobile phone revolution connecting the once cut-off continent.

This progress is the result of widespread democratisation. In 1980 there were just two African democracies – today more than 40 countries regularly stage multi-party elections.

Despite this, Africa faces common challenges. The first concerns growth in the number of people and where they will be living. Sub-Saharan Africa will be an urban continent by 2025, when one in four of the world's people aged under 24 will be from this region.

Sub-Saharan Africa's population has increased from 100 million in 1900 to 800 million today, and should reach 1.5-2 billion by 2050. And, as witnessed across north Africa, it is urban youth who have driven the revolutions.

This also highlights a second challenge: the imperative to create formal sector jobs.

At independence in 1964, Zambia had 3 million people; today it has more than four times that number. But formal employment has only risen from 300,000 to 400,000 in this time. Against a backdrop of rising unemployment, especially among the 3 million youths, and allegations of widespread corruption, politics is likely to get more fractious in the runup to this year's presidential election.

Growth per se is not enough, as Egypt illustrates, with perceptions of inequality and government failure to deliver able to bite politically.

Moreover, African elites still have other options – from aid, natural resource rents, and remittances – which discourage aggressive reforms and keep the middle class, the source of Tunisia's and Egypt's revolutions, small and vulnerable to politically directed favours.

A third challenge concerns the depth of democracy. Elections are frequent but the quality of Africa's democracy is patchy. Freedom House – a Washington NGO that studies democracy, political freedom and human rights – ranks most sub-Saharan African states in the "partly free" category given limits in respect for political, civil, and economic rights and liberties.

Countries where the ruling party won by more than 80% – including Equatorial Guinea, Ethiopia, Rwanda and Djibouti – were scarcely more democratic than those that resorted to outright repression or had no polls at all. What might discontented voters do where there is no honest parliamentary system through which to vent disapproval?

A final challenge is the extent to which African leadership is connected with its youthful population.

Leaders regularly cock a snook at democratic niceties in staying in power and many seem largely out of touch with their people's needs, behind their high walls and blue-light security cavalcades. Sub-Saharan Africa's leaders are less Facebook generation than facelift. While the average age of Africans is 25 (and Europeans 45), the ages of leaders is 70 (Africa) and 55 (Europe).

What is very puzzling is the tolerance of its people to the most egregious of leadership, from Robert Mugabe in Zimbabwe to Equatorial Guinea's Teodoro Obiang. The former's electoral and policy excesses are well-known; the latter's, who came to power via a bloody coup in 1979, less so, even if he was recently elected chair of the African Union.

Even though they regularly berate their colonial inheritance, Africa's leaders have shrewdly worked their people's ethnic, racial and linguistic divisions in their own favour. They have also kept their armies on a tight leash. In many cases, including Rwanda, Uganda, Eritrea, Ethiopia and Zimbabwe, these armies brought them to power in the first instance.

In Africa, internet connectivity, unemployment and anger at corruption and cronyism are all rising. Yet "internet revolutions" are made less likely by the use of tribe, family and religion to distribute favours, money and jobs.

To emphasise this point, Yoweri Museveni was elected in February for a further five years as president of Uganda, extending his rule to 30 years, the same as Mubarak.

These differences could quickly be transcended by the power of technology and the impatience of a new generation. Electoral systems that offer the prospect – but never the reality – of change, and the presence of mobile phones and youthful anger should make African autocrats increasingly uncomfortable.