More than two-thirds of refugees who claimed to be children turned out to be adults when they had their ages assessed, figures from the Home Office show.
The findings come after Tory MP David Davies said 14 teenagers who arrived in the UK from Calais on Monday did not "look like children".
Mr Davies' subsequent call for checks on the teeth of child migrants to determine their age was condemned by dentists as "inappropriate and unethical".
Data from the year ending in June reveals that 1,060 asylum applicants' ages were called into question. Of the 933 who were recorded as having an age assessment, 636 (68%) were deemed to be over 18.
A spokesperson for the British Dental Association (BDA) said: "We are vigorously opposed to the use of dental X-rays to determine whether asylum seekers have reached 18.
"It's not only an inaccurate method for assessing age, but it is both inappropriate and unethical to take radiographs of people when there is no health benefit for them."
Doctors of the World, which works in Calais, condemned Mr Davies's stance as "unethical, inappropriate and divisive" and called for disciplinary action to be taken against the MP.
However, the Monmouth MP defended his suggestion, saying authorities should not be "naive" about the issue of adults trying to get into the UK and instead they should be more "hard-nosed" to stop Britain being "taken for a free ride".
The "Jungle" refugee camp in Calais faces the prospect of imminent closure after a French court rejected an appeal from aid groups to delay the clearance.
French authorities are expected to empty the migrant camp in Calais in the coming weeks and dismantle it by the start of winter.