The case against the Tavistock and Portman NHS Trust was brought by Keira Bell, who started taking puberty blockers at the age of 16 but then “detransitioned”.
The trust runs the UK’s only gender identity development service for children.
Ms A, the mother of a 15-year-old autistic girl who is on the waiting list for treatment at the trust, also brought the legal action.
Lawyers for the pair argued children are "not capable of properly understanding the nature and effects of hormone blockers". Their barrister, Jeremy Hyam QC, said that the use of puberty blockers to tackle gender dysphoria did “not have any adequate base to support it”.
The trust’s legal team argued that puberty blockers are rarely used in children under the age of 13 and that they are a “safe and reversible treatment with a well-established history”.
Young people are made fully aware of the impact of hormone blockers prior to treatment and were “given all the necessary and appropriate information […] and very considerable support to assist them in their thought processes”, Fenella Morris QC, the trust’s representative, said.
Delivering the judgement, Dame Victoria Sharp, sitting with Lord Justice Lewis and Mrs Justice Lieven, said that children under 16 needed to understand "the immediate and long-term consequences of the treatment" to be able to consent to the use of hormone blockers.
The judges said in their ruling: “It is highly unlikely that a child aged 13 or under would be competent to give consent to the administration of puberty blockers.
“It is doubtful that a child aged 14 or 15 could understand and weigh the long-term risks and consequences of the administration of puberty blockers.”
They added: “In respect of young persons aged 16 and over, the legal position is that there is a presumption that they have the ability to consent to medical treatment.
“Given the long-term consequences of the clinical interventions at issue in this case, and given that the treatment is as yet innovative and experimental, we recognise that clinicians may well regard these as cases where the authorisation of the court should be sought prior to commencing the clinical treatment.”
Tavistock and Portman NHS Trust had previously said the claimants were trying to “impose a blanket exclusion” on under-18s being able to consent to medical treatment.