An online system trains machine learning models that, when applied to gathered sensor data, determines personal characteristics (e.g., age, gender, height) of an individual in a non-intrusive manner. Specifically, the online system trains a first machine learning model that analyzes sensor data gathered from a client device associated with the individual. The first machine learning model determines whether a trigger event, such as whether the individual is walking, is currently occurring. A second machine learning model trained by the online system analyzes sensor data corresponding to the trigger event to identify the personal characteristics of the walking individual.