The term comes from Gillick v West Norfolk and Wisbech Area Health Authority [1985] UKHL 7. The House of Lords held that a parent's right to decide about a child's medical treatment gives way once the child has sufficient understanding and intelligence to understand fully what is proposed. There is no fixed age: it depends on the child and on the decision.
Young people aged 16 and 17 are presumed able to consent to their own medical treatment under section 8 of the Family Law Reform Act 1969, so Gillick competence is mainly about under-16s.
What statutory guidance says. KCSIE 2026 doesn't explain Gillick competence. It lists guidance on Gillick competency and the Fraser guidelines under "Confidentiality" in Annex A's additional advice, as help with balancing children's rights and safeguarding responsibilities. Working Together 2026 doesn't use the term.
In school practice the idea is used when thinking about whether a pupil can make a decision for themselves, for example agreeing to a school nurse or counselling service, or whether a child's own wishes should carry weight. It does not let a child stop you acting on a safeguarding concern: KCSIE says staff should never promise confidentiality (paragraph 559), and safeguarding information can be shared without consent where needed (paragraph 146).
Not the same as the Fraser guidelines, which come from the same case but apply only to contraception and sexual health advice.