Hello! To add one more clarification to @paul2009's point: once you've enabled the feature, any orders fulfilled within the last 14 days from the day you enabled the feature will also trigger a review request email for those orders. This is a small technical detail. For the most part though, the feature is built to work for orders that come in after you've toggled this feature on, like Paul said
I've also noted your feedback on being able to reach previous customers with review requests. This is something we would like to eventually solve for, although realistically it is not something we will be able to prioritize in the near future.
Cheers, Karen