After not charging her rent for such a long time, I wouldn't present it to her as "I decided you should now pay rent" because she may think you are trying to get rid of her (unless you really do want her to just move out). Instead, I would "blame" it on inflation and present it as "As I'm sure you know, the price of everything has really gone up including our utilities, grocery bills, insurance, etc. It would really help out our household budget if you could give $xxx per month toward these expenses."
excellent point-this is also an optimal time of year to do so b/c (as the op could share with their dd) it's very common for a household to do a mid year financial review comparing the previous 2 quarters of the year's expenses to the prior year and making necessary adjustments.