Step 1: The Theme of the Poem.
The central idea of "My Mother at Sixty-Six" by Kamala Das revolves around the poet's feelings of concern and the inevitable passage of time. The poem is a personal reflection on the aging process and the fear of losing loved ones.
Step 2: The Poet's Emotions.
The poet expresses her concern for her aging mother, observing her frailty as she ages. The poet is faced with the painful reality that time takes its toll on everyone, and the emotional struggle of seeing a loved one grow old.
Step 3: The Universality of Aging.
While the poem is personal, it also resonates with the universal theme of aging. It highlights how time can make people vulnerable, and how the fear of loss is part of the human experience.
Final Answer: \[ \boxed{\text{The central idea of "My Mother at Sixty-Six" is the poet's concern for her aging mother, the inevitable process of aging, and the emotional struggles associated with the passage of time.}} \]