Parlay really shines when are at the age when students are going beyond the “basics” of understanding course material, and starting to form their own ideas and opinions. Typically, this happens some time in middle school (grade 6, 7 and 8) around the age of 12 or 13. But of course, everyone is different!
If you're familiar with his work, John Hattie talks a lot about “Piagetian Problems” in his Visible Learning research. According to Jean Piaget’s theory of cognitive development, in the the “Formal Operational Stage” (12 years old and onwards): Children and adolescents develop abstract thinking and are able to perform hypothetical and deductive reasoning. You may find it useful to think of Parlay as a tool that helps students best once they reach this stage.
Hope this helps :)