Reza’s story is unusual because of his age. But it illuminates a quiet corner of Europe’s migrant crisis: In the human tide washing up on the Continent, tens of thousands are children and teenagers who arrive on their own.

Last year, more than 23,000 unaccompanied minors applied for asylum in the 28 member countries of the European Union, according to the United Nations. That was before the number of refugees surged this year. By now, 30,000 are estimated to live in Germany alone.

Two dozen psychological profiles of recent arrivals, compiled by the authorities and seen by The New York Times, reveal patterns: Many of Europe’s new mystery children are boys ages 14 to 17, sent by families too poor to pay smugglers for more than a single journey. Some lost their parents to war or murder at home. Others, like Reza, were separated from them in the chaos along the way. A few had their own reasons to flee: escaping recruitment as child soldiers or suicide bombers.