The English use of "they" to refer to a singular referent that is non-specific or of unknown gender dates back to the 1300s. Recently, "they" has emerged as the dominant pronoun for individuals who identify as gender nonbinary, and a coherent subset of English speakers (innovators) explicitly accept "they" when referring to a specific individual of known gender ((i) “Sarah slept because they were tired”) in off-line judgments. The present work examines whether on-line comprehension difficulty patterns with off-line judgments. Using a maze-task, participants' read sentences like (i) containing “they” with plural or singular subjects, or “he/she” with named subjects that matched or mismatched in their typical gender. Gender mismatched evoked reading difficulty. Plural “they” also caused difficulty, but significantly less. Innovators experienced less difficulty with singular “they” than non-innovators, but still processed plural “they” faster than singular “they”. This work suggests that the grammar of “they” may have yet to stabilize even for innovative speakers.