Heart transplantation (HTx) is a life-saving procedure in patients with end-stage heart diseases. However, organ rejection is still a major issue following HTx and a major cause of death in heart transplant recipients (HTRs). Currently, endomyocardial biopsy (EMB) is the gold standard to monitor for heart transplant rejection, although it is an invasive, expensive, and traumatizing procedure, particularly in children. Therefore, designing a non-invasive method such as a simple flow cytometry-based assay to detect immune biomarkers of heart rejection would represent a major step forward to identify and minimise HTx rejection. This approach may improve patient outcomes and reduces healthcare costs. To assess changes to the immune system in HTRs, we analysed 250 PBMCs samples from 37 HTx recipients, aged 6 months to 18 years old using high-dimensional flowcytometry. In each patient, we assessed >60 populations of immune cells both before (pre-Tx) and after (post-Tx) HTx. As age can profoundly affect the composition of immune system, we divided patients into two age groups (<2 y and 10-18 y) for further analysis and compared them with age-match healthy controls. We found that several subsets of T cells such as MAIT cells, gd T cells, and NKT cells changed in peripheral blood, not only between pre-Tx and post-Tx timepoints, but also between the two age groups. These findings identify potential immune biomarkers of heart rejection in children based on age, which may lead to personalized treatments to prevent organ rejection and enhance the survival of heart transplant recipients.