As far as I am aware, there is no minimum age requirement but obviously common sense must be applied.
A minor can witness a will, but not a young child since they will not be capable of understanding the importance of witnessing the will’s execution. The witness must have the requisite mental capacity ( and not be be blind).
The witnesses may be called to give evidence after death and should therefore be of a suitable age and competent to give evidence. The point of having witnesses is so that they can speak after the testator’s death if queries arise.
S9 Wills act 1837 and its substitution from the Administration of Justice Act 1982.
No mention of age.
Age 17 ½ should be fine.
For Wills, in reference to age 18, it is only that Wills made by a person under the age of 18 are not valid ( apart from the armed forces / sailor at sea exemption Wills (Soldiers and Sailors) Act, 1918.)
S. 9 substituted by Administration of Justice Act 1982 (c. 53, SIF 116:5), ss. 17 , 73(6)
17 Relaxation of formal requirements for making wills.
The following section shall be substituted for section 9 of the Wills Act 1837—
9 Signing and attestation of wills.
No will shall be valid unless—
(a)it is in writing, and signed by the testator, or by some other person in his presence and by his direction; and
(b)it appears that the testator intended by his signature to give effect to the will; and
©the signature is made or acknowledged by the testator in the presence of two or more witnesses present at the same time; and
(d)each witness either—
(i)attests and signs the will; or
(ii)acknowledges his signature,
in the presence of the testator (but not necessarily in the presence of any other witness),
but no form of attestation shall be necessary.”.
Andre Davidson
Finantium