The age of majority in Britain, which generally includes eligibiligy to enter into a contract, is - as it is in most countries - 18.
However, any 'verbal contract' implied by the JW baptism questions is invalidated by its terms regardless of the candidate's age, because the claim that their 'organisation' is 'spirit directed' is semantically ambiguous, and based on flawed chronology and stupid superstition.
As such, the Watch Tower Society / Jehovah's Witnesses have no legal right to make any JW members or former members do (or not do) anything at all, regardless of age. It is only their use of peer pressure (including convincing other members to shun dissidents) that 'allows' them to do so. The 'commitment' made to the 'organisation' has no legal standing.
Because baptism is essentially meaningless in legal terms, it doesn't really restrict a minor from undergoing the ritual.