This paper presents a downlink scheduling framework for Extended Reality (XR) applications in 6G networks. Building on the 3GPP Release 18 Protocol Data Unit (PDU) Set-based quality of service (QoS) framework, we propose a PDU Set Delay Tracking Queue (PDTQ) mechanism that provides real-time visibility into each PDU Set's remaining delay budget. Unlike existing approaches relying on indirect urgency metrics, PDTQ directly tracks PDU Set aging across queue positions for precise identification of delay-critical data. Leveraging this, we develop the PDTQ-Guided XR (PiXR) scheduler, an adaptive heuristic that allocates resources based on delay criticality and instantaneous channel conditions. Simulations show that PiXR achieves approximately 4--5x higher satisfied user counts than state-of-the-art PDU Set-aware schedulers under limited resources, while maintaining comparable or better performance under adequate conditions.