Last month, Kent County Council said it is so overwhelmed by the number of asylum-seeking children, it has run out of capacity to care for local youngsters.
Between January 2022 and June this year, 23 migrants claiming to be “children” had their ages reassessed.
Of these, 15 were found to be adults, Freedom of Information Figures show.
A source said: “Migrants lying about their age has always been an issue, but the smugglers are now actively briefing them to say they are under 16 because they will receive better treatment.
“They have no documents on them when they arrive in the UK and you can’t physically determine someone’s age.
“Some countries use dental examinations, but that’s not definitive.
“How old someone looks depends on their growth rate.”
Natalie Elphicke, MP for Dover, said: “Grown men posing as children in our schools and children’s care homes puts kids at risk.
“Urgent action is needed for compulsory age checks on illegal migrants who claim to be children.”