Once they get there, the older ones may be subjected to physical assessments to determine whether they are in fact still children who qualify for certain rights.
The problem is, the tests are flawed, a biologist alerted Monday in the journal Annals of Human Biology. Many minors risk being wrongly classified as adults.
"The discrepancy, no matter how small, has life-changing consequences," said Noel Cameron of the Loughborough University in England.
A mistake "results in the loss of any access to rights and privileges afforded to children, including housing and foster care, and may lead to repatriation and continued persecution."
Cameron pointed out flaws in the so-called "skeletal maturity" assessment of the hand and wrist bones of teenagers, used by many European countries to determine the age of those without papers.
On average, about 50 percent of European boys have adult skeletons already by the age of 16-and-a-half, he said.
Conversely, about one in five are not yet "skeletally mature" by the age of 18.
"Thus a decision based on adulthood being defined as the attainment of full skeletal maturity condemns those skeletally advanced 16- and 17-year-olds to laws governing adults, and those skeletally delayed 18-year-olds to laws governing children," Cameron wrote.
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Using a physiological measure to determine an asylum-seeker's age, and thus their fate, "is inappropriate at best and simply wrong in over one third of assessments," he added.
In Britain alone, there were 2,168 asylum applications from unaccompanied children in the year ending in June 2015, said the article -- some eight percent of the total applications submitted in that period.
Age assessments were carried out in 488 cases, of which 58 percent resulted in an adult classification, though it is impossible to know how many would have been wrong.
"Figures similar to these can be found in most countries of the European Union," wrote Cameron.
Almost all, he added, used skeletal maturity tests as the main age gauge of migrant asylum-seekers.
"It is scientifically indefensible to ignore the known imperfect association between maturity and age in order to decide who will, or will not, be granted the opportunities afforded by asylum in Europe," said Cameron.
"Chronological age is not a biological variable. It is a social construct relating to behavioural maturity and thus behaviour ought to be the core target of assessment," the biologist told AFP by email.
According to the UN refugee agency, more than 74,000 migrants have already arrived in Europe by sea this year from countries like Syria, Afghanistan and Iraq, many risking their lives in dangerous vessels operated by people smugglers.
Last year, the total was over a million -- almost a third of them children.