Answer is The narrator is looking at her reflection in a mirror.
Reason
“My image stood before me, acted on by fifty years."
✅ The narrator is looking at her reflection in a mirror.
This fits the statement, “my image” says it’s her, not a twin, not someone else.
❌The narrator is looking at her identical twin.
❌The narrator is looking at an image of someone who stood up before she did.
❌The narrator is looking at someone who looks like her but older.