To your point, she is getting old enough where she needs to hire someone for help, and her house is empty. So she's clearly 45+. A sturdy farm woman simply wouldn't need the help if she were under 45. She would employ locals and not need someone to stay on the ranch to help her.
BUT...her dress still fits. It isn't a new dress, it was just 'something she hadn't worn in quite while'. This means she could still fit into the dress and enough womanly form to fill it out, so to speak. This means she couldn't have been over 65. She's also young enough that a fit, and vigorous ranch hand finds her attractive.
50-60 is the final AND CORRECT answer.