An interesting conundrum is posed by a comparison of these two recent authorities, both cases where Lambeth wished to refuse or discontinue s20 accommodation to a 'child' who they considered was in fact an adult. It seems that in some cases a Local Authority is the arbiter of whether or not a child is in fact a child and in some cases the court is the arbiter.
So, R (on the application of M) v Lambeth London Borough Council and R (on the application of A) v Croydon London Borough Council  All ER (D) 273 (Jun)  EWHC 1364 (Admin) (20 June 08) tells us that the Local Authority are perfectly entitled to decide that a child is in fact an adult and no s20 Children Act 1989 duty to accommodate arises.
But, The London Borough of Lambeth (Appellants) v TK, by her Litigation Friend (First Respondent) v KK (Second Respondent)  EWCA Civ 103 (21 Feb 08) tells us that where family proceedings are extant and a s37 Children Act 1989 report is ordered by the court, it is not open to the Local Authority to unilaterally decide that the child is an adult and that no s37 duty to consider applying for a care or supervision order under s31 arises. In that case Lambeth had reported that since in their view the child was not a child they need not consider s31 orders and would be terminating s20 accommodation forthwith. That was the wrong approach because in the context of family proceedings and pursuant to s37 it was for the court to determine whether or not there was a child to whom a s37 direction or s31 order could apply.
However, lest it be thought that the solution in this type of case (usually where a child is an asylum seeker without papers) the answer is to get children act proceedings afoot before trying to obtain s20 accommodation - the judgment in the latter case makes clear that family proceedings cannot be instigated or continued as a mere contrivance in order to expand the courts powers to review a local authority's decision about age. In the normal course of events a Local Authority's decision that the child is an adult can be challenged only by Judicial Review on the usual Wednesbury unreasonableness etc grounds.