1. A computer-implemented method for generating a user classifier model, the method comprising: for each viewer of a plurality of viewers of digital videos, the viewers including viewers from a plurality of different households, and for a demographic attribute: identifying a value of the demographic attribute for the viewer; identifying a period of time during which the viewer viewed digital videos via a video hosting service; identifying a plurality of digital videos viewed by the viewer during the identified period of time; for the identified period of time, determining a set of features corresponding to the viewed digital videos, the features comprising content features derived from audiovisual content of the plurality of digital videos viewed during the period, and view-block features related to a division of the plurality of digital videos into subsets of videos viewed within a threshold amount of time of each other; and associating the determined set of features with the demographic attribute value; and for each demographic attribute value of a plurality of the demographic attribute values identified for the demographic attribute, training, by a computer processor, a user classifier model for the value of the demographic attribute using the sets of features associated with the demographic attribute value as input for an ensemble classifier training algorithm.