Humans in good mental health and of reasonable intelligence are organisms capable of comprehending their own existence, and of deciding on when that existence becomes sufficiently burdensome that it is no longer desirable.
No one else, no society, no religious group, no government, has any right to interfere in that decision.
Whether or not a 'terminal illness' is involved is irrelevant; we are all going to die, it's just a matter of time.
Before ending his (or her) own life, however, the person has to consider the impact of his decision on those who are dear to him, and to whom he may be dear, and any who may be dependent upon him (children, spouse).
The relevant questions he must ask are:
Advanced age and deteriorating health and fitness may well be a factor in whether a person decides he has had enough.
Living with pain is pretty miserable.
- Am I enjoying life?
- If not, is that a temporary condition? Depression? Will the condition improve with time?
- Am I a net asset to, or burden on, society and those around me?
- What impact would my suicide have on my family? Can I prepare them for that?