One in four people alive today is a young person aged 15 to 29: that’s nearly 1.8 billion in total, of whom close to 90% live in developing countries.

Demographically speaking, the next couple of decades are a unique window of opportunity. With the exception of Africa, the world is ageing, which means the proportion of young people in the global population will never again be so high.

The “youth bulge” – broadly defined as a peak in young people’s share in a country’s population – could be a blessing for many countries, provided governments and others do what is necessary to ensure young people become healthy, educated and productive citizens.

But where should they be looking to find the data that will help them develop more evidence-based policies and programmes? Credible data on young people is rather difficult to come by. Data split by gender is even harder to find.

A youth development index (YDI) developed by the Commonwealth Secretariat seeks to address this anomaly. The index, which aggregates data on 18 indicators related to the health, education, employment and political participation levels of young people, provides a snapshot of the multi-dimensional nature of youth development in 183 countries.

Like all composite indices, the index has its shortcomings. But crunching the data it throws up offers a lot of food for thought, particularly for policymakers. Overall, the prospects for a majority of young people in the world are bleak. Three in four of them are failing to get the start in life they deserve.

The inequality in opportunities and outcomes for young people across countries is huge. For instance, the youth mortality rate is on average five times higher in the lowest-ranked countries of the index in comparison with countries where young people have the best shot at living a decent life. Equally shocking is the gap in secondary education enrolment rates for the highest and lowest ranked countries.

In some respects, however, the best and worst-performing countries are similar: unemployment, drug abuse and mental disorder rates among young people are equally worrying in both.

It is the poor quality of healthcare and education in so many countries, and a socio-economic horizon blighted by poverty, unemployment, violence, inequality or lawlessness, that are usually to blame. For girls and young women, the future looks particularly precarious.

Only 40% of countries have achieved something close to gender parity in secondary education enrolment rates. In tertiary education, that figure drops to 4%. Young females are on average twice as likely as young males to be out of education, employment or training. Worldwide, HIV prevalence rates are also significantly higher among young females.

The study reveals that large youth bulges, violent conflict, a country’s income status and regime type are among the most important predictors of youth development. Nearly all the countries in the index where youth development is categorised as medium or low are ones where young people make up a significantly large proportion of the population.

But the unluckiest young people are the ones living in countries that have also been ravaged by violence and armed conflict – in other words, the countries at the bottom of the index, such as Central African Republic, Afghanistan, the Democratic Republic of the Congo and Chad.

In a similar vein, political unrest and war in the Middle East and North Africa are undoing the significant progress many countries in the region had made in the past few decades in providing decent education and healthcare to their young people. Syria, for instance, has registered the largest drop in education scores in the five-year period tracked by the index.

A critical factor in youth development is the extent to which young people are able to participate in the political and civic life of their countries. But role models are in short supply: according to the Inter-Parliamentary Union, of the 45,000 national level legislators in the world, only 1.9% are below the age of 30. It is hardly surprising that young people are fast losing faith in apathetic political systems at a time when social media is giving them the tools to launch an insurgency from their bedrooms.

In 2015, the world signed up to the sustainable development goals, an ambitious set of targets to be achieved by 2030. Be it ending poverty or pushing back climate change, getting close to these goals will depend largely on the ability of today’s youth to rise to the challenge.

The evidence suggests most countries are failing to prepare their young people for the economic, environmental, technological and political upheavals that lie ahead. In much of the world, the window of opportunity for converting the youth bulge into a “demographic dividend” will remain open for another few decades at best.

Unless the challenges young people face are treated as urgent, it is more than likely that many among their nearly 2 billion-strong number will end up as a lost generation.

Abhik Sen and Rafiullah Kakar are the co-editors of the 2016 Global Youth Development Index and report published by the Commonwealth Secretariat.