A Study to Investigate the Efficacy, Pharmacokinetics, and Safety of Mitapivat in Pediatric Participants With α- or β-Non-Transfusion-Dependent Thalassemia

Inclusion Criteria: * Written informed consent/assent from the participant (or their legally authorized representative, parent(s), or legal guardian) must be obtained before any study-related procedures are conducted and participants must be willing to comply with all study procedures for the duration of the study. * Aged 1 to \<18 years and weighing at least 7 kilograms (kg) at the time of providing informed consent/assent. * Documented diagnosis of thalassemia (β-thalassemia with or without α-globin gene mutations, hemoglobin E (HbE)/β-thalassemia, or α-thalassemia/hemoglobin H (HbH) disease) based on Hemoglobin (Hb) electrophoresis, Hb high-performance liquid chromatography, and/or DNA analysis from the participant's medical record. If this information is not available from the participant's medical record, the test(s) can be performed by a local laboratory during the Screening Period. If a local laboratory is unable to perform the test(s), results from the comprehensive α- and β-globin genotyping performed by the study central laboratory can be used. * Hb concentration ≤10.0 grams per deciliter (g/dL) \[100.0 grams per liter (g/L)\], based on an average of at least 2 Hb concentration measurements (separated by ≥7 days) collected during the Screening Period. * Non-transfusion dependent, defined as ≤5 transfusion episodes (also referred to as "transfusion events") during the 24-week period before randomization and no red blood cells (RBC) transfusions ≤8 weeks before provi…