There is no definitive answer to this question as it depends on a number of factors, including the definition of youngest parents. However, according to a 2013 UNICEF report, the country with the youngest average age of first-time mothers is Niger, where women give birth for the first time at an average age of 15.4 years old. This is followed by Chad (15.9 years old), Mali (16.1 years old), and Somalia (16.3 years old).
It should be noted that these statistics are based on averages, and there are certainly many young parents in other countries as well. For example, in the United States, the teen pregnancy rate has been declining in recent years but there are still about 700,000 teenage girls who become pregnant each year. This means that there are still plenty of young parents in America despite the overall trend towards later childbearing.