In this work, we study the age-optimal data collection problem in UAV-enabled wireless sensor networks, where a UAV is dispatched to collect data from the ground sensor nodes (SNs). The age of information (AoI) of data collected from each SN consists of the data uploading time and the time elapsed since the UAV receives the data. Based on this definition, a combinatorial optimization problem is formulated to minimize the maximum AoI among all the SNs. To solve this problem, we propose a joint SN association and trajectory planning strategy to strike a balance between the SNs' uploading time and the UAV's flight time in various scenarios. In particular, SN association selects appropriate collection points (CPs) at which the UAV hovers to collect sensed data based on the affinity propagation (AP) method, and schedules which SNs to upload at each CP. Trajectory planning is then performed to find the max-AoI-optimal trajectory along the CPs using dynamic programming (DP), where the max-AoI-optimal trajectory is shown to be a shortest Hamiltonian path. Simulation results validate the effectiveness of the proposed strategy, and show how the UAV's trajectory is affected by the AoI metric.