Generally, any person over the age of 18 years who has legal capacity can make a Will.

Legal capacity is a question of the ability to understand and make reasoned decisions about your affairs and may vary depending upon the transaction or decision involved. For example, a person may have the legal capacity to purchase land but not the requisite legal capacity to make a Will. It is also the case that a person may sometimes have legal capacity and sometimes not, perhaps due to a medical condition, changing medication, stress, or grief.
It is possible to make a Will, notwithstanding that the will maker cannot read or write, cannot speak English, is deaf or blind, or has any other physical disability, provided the correct execution clause is used and provided the will maker otherwise has legal capacity.

It is also possible to make a Will even where there is a diagnosis of Alzheimer’s, dementia, or other mental illness, provided that the will maker has sufficient capacity to understand the effect of making the Will. Generally, this means that the will maker should understand the extent of their property, be able to give instructions about how the property is divided, and realize the effect of those instructions on those who claim an entitlement to their property. The question of legal capacity when making a Will is usually assessed by the solicitor taking instructions, who may also request medical evidence from a suitable professional to assist in making the assessment.

It is not possible to delegate the Will making process to another person, so, for example, your appointed attorney cannot make a Will on your behalf… so make one while you can.