This study investigates knowledge of gender agreement in Spanish L2 learners and heritage speakers, who differ in age and context/mode of acquisition. On some current theoretical accounts, persistent difficulty with grammatical gender in adult L2 acquisition is due to age. These accounts predict that heritage speakers should be more accurate on gender agreement than L2 learners, because their Spanish language acquisition started in infancy. Sixty-nine heritage speakers, 72 second language (L2) learners, and 22 native Spanish speakers were tested on their oral production, written comprehension, and written recognition of Spanish gender agreement. Results showed advantages for L2 learners in written tasks but advantages for heritage speakers in the oral task. We discuss the significance of these findings for SLA and heritage language acquisition.