been under a mistake all my life

"I don't know about that, ma'am. I've been a sailor, man and boy, forty years, and here I am, well and hearty to-day."   "The captain is about your age, isn't he, Aunt Rachel?" said Jack, maliciously.   "I'm only thirty-nine," said Rachel, sharply.   "Then I must have been under a mistake all my life," said the cooper to himself. "Rachel's forty-seven, if she's a day."   This remark he prudently kept to himself, or a fit of hysterics would probably have been the result.   "I wouldn't have taken you for a day over thirty-five, ma'am," said the captain, gallantly.   Rachel actually smiled, but mildly disclaimed the compliment.   "If it hadn't been for my trials and troubles," she said, "I might have looked younger; but they are only to be expected. It's the common lot."   "Is it?" said the captain. "I can't say I've been troubled much that way. With a stout heart and a good conscience we ought to be jolly."   "Who of us has a good conscience?" asked Rachel, in a melancholy tone.