They are more likely to have vices and drop out of school than anywhere else in the UK.
Welsh 15-year-olds, according to an Assembly government report, smoke more cannabis and drink more alcohol than their peers in England, Scotland and Northern Ireland. Teenage girls also have a higher rate of pregnancy, and there are more reported cases of sexually transmitted diseases overall.
According to the 2008 Children's and Young People's Well-Being Monitor for Wales, 11 to 15-year-olds are also less likely to say "they like school a lot" compared with those in England.
But there is a glimmer of hope in the 268-page report published last week for teachers and schools; more teenagers are leaving school with at least one qualification than a decade ago, and 11-year-olds are outperforming their state school counterparts in England.
The document explores how far Wales has gone in meeting targets of the United Nations Convention on the Rights of the Child. It draws together evidence and statistics in seven areas, including education, health and leisure time.
Assembly government ministers said the report - the biggest and most reliable to be compiled in Wales - showed where Wales had progressed and needed to improve.
But Keith Towler, children's commissioner for Wales, said the document needed to be acted on quickly for the nation to step to the mark. He was particularly critical of poverty rates; 29 per cent of under-18s still live in poor homes.
"If we are going to succeed in making Wales a place where children and young people can prosper, we had better make sure we step up to the mark and identify solutions that make a difference instead of sitting on our laurels - and on another document," he wrote in Western Mail this week.
Elsewhere in the report, a bleak picture was given of parenthood. Mothers in Wales are more likely to smoke during pregnancy compared with the rest of the UK. They are also slightly less likely to read to their young children.
Teenage girls are less likely to use a condom during sex. Girls also become increasingly dissatisfied with their lives as they age, whereas boys stay the same.
But a rise in cannabis smoking is affecting boys and girls equally; 30 per cent of 15-year-olds have tried it, more than the rest of the UK.
Assembly government ministers conceded there has been progress following publication of the report this week, but there is a long way to go in catching up with the rest of the UK.
Speaking in her capacity as children's minister, Jane Hutt said the wellbeing of children and young people was at the heart of the government's policy: "There are issues that still need our attention."
IN NUMBERS: WHAT THE MONITOR REVEALS
- 66 per cent: Pupils in Wales who say that they have never been taught about their rights under the UN Convention on the Rights of the Child (UNCRC) at school.
- 50 per cent: Young people who think their classmates are kind and considerate.
- 50 per cent: Young people who know who represents them on their school council.
- 80.3 per cent: Children at KS1 who achieve the core subject indicator in English, maths and science who are not eligible for free school meals.
- 62.6 per cent: Children who achieve the above and are entitled to free school meals.