Underage, unaccompanied migrants trying to reach the UK and France are particularly vulnerable to being deported.
As many as 67% are aged between 16 and 17 when they arrive but the extension of statutory deadlines makes them easy to expel when they lose their minor status aged 18.
"We had a young man who was picked up by police and taken to a detention centre," said Michael McHugh from the charity Help Refugees.
"He explained in his native language that he was 16 but he returned the next day with a piece of paper, where it was stated that he had been interviewed in Amharic (the official language of Ethiopia), not his first language.
"And they basically changed his date of birth, so it was written that he was over 18."
According to the Home Office, at the end of 2016, the number of unaccompanied children who had been waiting for an initial decision on their asylum application for more than six months increased by 143%.
If the decision is not made within six months of the application date, the secretary of state must inform the applicant. However, some minors have been waiting for more than two years.
"If the decision is made after they are 18, then they are no longer a child, and unless they succeed in their claim on its own merit, they are not going to be given protection as a child," said Zofia Duszynska, a lawyer at Duncan Lewis in London.
'I considered ending my life several times'
This negatively affects their mental and physical health, according to Mina Fazel, a child psychologist specialising in the mental health of refugee children.
Delays impact upon their ability to learn and can cause a range of anxiety disorders.
Cédric, a young refugee living in France, was legally recognised as a minor two and a half years ago but recently turned 18 and therefore lost his minor status.
"They told me I have to leave the children’s home," he said.
"I have nowhere to go, I think I will go back living on the street. I considered ending my life several times.
"I told myself if I am bound to spend the rest of my life like that in France, I might as well end it."
How can you determine a child's age?
Age determination is therefore key for the wellbeing of minors but even the most precise technique on offer still has an age bracket of a year and a half.
There is no European or French standard for telling a child's age, Delphine Tardivo, an odontology (the scientific study of the structure and diseases of teeth) specialist, explained.
"We used wrist x-rays, but this was not reliable.
"We also used wisdom teeth but the problem is that wisdom teeth can grow at very different ages. Some individuals have wisdom teeth at 15, and others at 30.
"This method is still used, but there is a high level of uncertainty.
"There is no technique that is precise enough to assess without any doubts if the individual is underage or not."
There are no standardised physical charts for children of all nationalities, added Zofia Duszynska
"The age assessment process in Western Europe and all the guidelines for bone development and educational development are based on western children who grew up in a western environment," she said.
In the UK and France, reports of deadlines being extended without explanation are rife. Meanwhile, teenagers like Cédric are left in limbo.
Find out more about Cédric's story and the challenges teenage migrants face when seeking asylum in the UK and France in the above video player.