Well this is embarrassing.
… the Secretary of State’s own department drafted the legislation which she is now saying is anomalous and/or absurd and these provisions must have been drafted at a time when her officials must have been aware of the alleged problems caused by the use of the word “child” from previous cases.
The quote is from paragraph 43 in the recent case High Court case of R (AA) v SSHD and Wolverhampton City Council  EWHC 1453 (Admin). The case concerned the definition of “a child” under recent immigration laws that restrict the detention of unaccompanied children to a maximum of 24 hours at a short-term holding facility.
The Claimants argued that whether or not a person is a child is a matter of objective fact.
Theresa May’s department argued, without any basis in the legislation that I can find, that whether a person was a child should be determined by what an immigration officer “reasonably believed”.
The Secretary of State said that the Claimant’s argument produced an absurd and unworkable outcome, as there was no way the Home Office could obtain objective evidence of a person’s age within the 24-hour time limit. If objective evidence was required, the Home Office would have to release many persons who claimed to be children but who were reasonably suspected to be over 18.
Sir Stephen Silber rejected the Secretary of State’s submissions and reminded her that she had drafted the laws herself. Despite being fully aware of the complications from previous case law on age-assessment, she had not drafted the definition of a child as requiring only “reasonable belief” on the part of an immigration officer. If that was the government’s intention, Parliament would need to change the legislation, not the High Court (see §§ 42 to 45).
For now, the 24 hour limit must be applied in cases of detained children (or claimed children) regardless of the Secretary of State’s beliefs as to the person’s actual age.
Michael Armitage appeared for the Claimant, instructed by Stuart Luke of Bhatia Best.
[UPDATE: According to a post by Monckton Chambers, permission to appeal to the Court of Appeal has been granted by the Judge, Sir Stephen Silber].
 Paragraph 18B of Schedule 2 to the Immigration Act 1971, as inserted by section 5(4) of the Immigration Act 2014.