[IL] Asking my employee about retirement plans

I have an employee who is in her mid 70s and she is just not good at her job. She has been on my team for about three years and I have given her moderate performance reviews because she does the basic functions of the job, but there is a lot of babysitting still. At this point I need someone who can take initiative and can be more self sufficient (I have told her this and included it in her goals for this year. I plan to give her needs improvement at the next review.

I want to ask her when she plans to retire because if it’s soon I don’t want go through the whole process of putting her on a performance improvement plan and terminating based on that. She’s a very nice lady and I don’t want to embarrass her, but I need someone I can count on and don’t feel the need to micromanage to ensure things run smoothly. I’d like her to leave on her own terms. I have asked her things like what is your three year plans to try and get a sense of whether retirement is even on her radar and she did not take the bait. She said just to continue to grow in her role. She also made a comment one time that most people think at her age she should be retired and be a lady of leisure lol. Help!