From Law wiki, the wiki for law research

A person is automically exempt from criminal liability if he or she:

• is under 10 years of age, or
• is classified as insane

A person may be shown to be exempt from criminal liability if he or she:

• is under 14 years of age, and has not developed a
• sufficiently advanced understanding of right and wrong to merit liability, or is insufficiently responsible for his or her actions.

In all these cases, the accused may be liable to detention, e.g., under the Mental health act (1983), but cannot be convicted of the offence.