The actor spoke to the Sunday Times about his decision in February to become a patron of Dignity In Dying, a campaign organisation that supports the option for mentally competent individuals with a terminal illness to take their own lives.
The X-Men star, whose dedication to the cause was strengthened after being diagnosed with coronary heart disease five years ago, explained: "A lot of it has to do with my age. I was 70 last year and there is something about achieving threescore years and ten, isn't there?
"Then I had had a family member who had been very ill and quite recently I'd heard the story of an illness and a death... It so shocked me that I decided to come out about my membership of Dignity in Dying."
When asked about the tragic passing of his loved one, Stewart said: "I'm reluctant to go into detail. Enough to say this person was driven to an extreme situation of ending their life in the most ghastly way... There has got to be an alternative when someone is suffering so badly and is ready to go."
He added: "I have the strong feeling that, should the time come for me, having had no role in my birth I would like there to be a choice I might make about how I die."
Stewart was awarded a knighthood in the 2010 New Years Honours List, an achievement that he later claimed to be "stunned" by.