It is possible for individuals aged 18 or above and of sound mind to execute a Will; though, there are special provisions that allow members of the armed forces to make a Will even if they are younger than 18.
According to the Mental Health Act 1983, the Court of Protection may authorise someone who is mentally incapable of making a Will on their own. For further guidance regarding the creation of a Will for an individual with mental incapacity, one should refer to the Public Guardianship.