The Mental Capacity Act (MCA) reflects that 'capacity' isn't a simple matter. A key principle is proportionality: for example, a carer isn't expected to spend too long worrying about whether or not someone who (for example) has an intellectual disability or dementia has capacity to decide whether they'd prefer tea or coffee.
However, what about capacity to engage in a sexual relationship? That's one of the most common reasons that I've been asked to give an opinion on capacity, as a Clinical Psychologist specialising in working with people with intellectual disabilities and/or autism spectrum conditions.
Deciding whether or not someone has capacity to decide whether or not have an abortion is another issue that involves the courts in complex, often distressing cases - particularly when the individual in question is found to lack capacity to decide whether or not to have an abortion, in which case the court has to decide (with the help of people who know the individual well and usually with the assistance of expert witnesses) whether or not it's in the person's 'best interest' to have an abortion.
Similar heart-wrenching debates (to which there's often no obvious 'right or wrong' answer) will also have to take place if assisted dying is made legal: simply saying that 'only people with mental capacity' can opt for it is to ignore a raft of existing legislation that would intersect with any assisted dying legislation. For example, if assisted dying is deemed to be a good thing for someone who has capacity to opt for it, why should people who lack capacity to opt for it be denied it?
There's also the Mental Health Act (MHA) to consider - and in practice the MCA and the MHA often come into conflict as it is! So, if someone with a history of significant psychological ill-health says that they want to end their life, I imagine that the court will want an opinion on that person's current state of mind. That will probably also apply to older people, if there's any suggestion of dementia.