The Local Government Ombudsman said the authority had failed to refer the man, known as Mr B, to a National Referral Mechanism (NRM) for identifying potential victims of modern slavery.
The report also criticised the authority for interviewing him in a police station, failing to provide evidence of an appropriate adult being present, and failing to provide him with a formal copy of the result of their decision and how he could potentially challenge it.
The LGO investigation followed a complaint on Mr B’s behalf by Ms C, who said the council’s decision was “unlawful” due to the alleged failures.
Mr B came to the UK in 2017 with, Ms C said, several other people who were picked up by the police at the same time.
At the time Lincolnshire County Council determined he was over the age of 18, however, Ms C said he was a child at the time and could not complain about the assessment.
“The guidance is clear a police station is not a suitable place for an age assessment,” said the LGO’s report.
“If the council had acted as it should have done, it is likely Mr B would have been interviewed at an alternative setting with an appropriate adult present and would have been informed of how he could challenge the decision.”
The LGO, however, did not speculate about whether the outcome would have been different.
It added: “Failure to refer to the national referral mechanism meant Mr B possibly missed out on some support as a likely victim of human trafficking.
The council argued that the guidance was not statutory, however, the ombudsman said it should have followed it unless there was good reason not to – something the LGO said was not present in this case.
The report said the council had argued an appropriate adult was present and a signature was on the form, however, the LGO said it was not in the correct section and four signatures on the paperwork were for Mr B, his interpreter and the two social workers present.
The LGO did say, however, that questions were in line with guidance and that Mr B had opportunity to rest and receive food prior to being interviewed.
The LGO told the council to pay Mr B £250 compensation and to apologise to him.
The authority was also requested to remind officers of the guidance and provide evidence that training had been undertaken.
Jo Kavanagh, Assistant Director of Children’s Services, said: “We fully acknowledge and accept the findings in this case which was from four years ago.
“Following national developments, our practice has changed and improved to guard against anything similar from happening in the future.
“We are confident that our social workers are well trained and supported and that our practice is now solid and robust to ensure the issues and failings recognised in the report would not occur again.”