The present study investigated developmental changes in lexical production skills in early sequential bilinguals, in both Spanish (L1) and English (L2), exploring the effects of age, years of experience, and basic-level cognitive processing (specifically the ability to resist interference) within a timed picture-naming task. To assess resistance to interference, naming was compared in low competition (blocked-single language) vs. high competition (mixed-alternating language) conditions. Participants were 100 individuals, 20 at each of 5 different age levels (5-7, 8-10, 11-13, 14-16, & young adults). All had learned Spanish as a first language in the home, with formal English experience beginning at 5 years. Gains were made in both languages across age. However, there was a developmental crossover from Spanish dominance in the youngest children, through a period of relatively balanced Spanish and English skills in middle childhood, culminating in a clear pattern of English dominance among adolescents and young adults. Although all groups experienced a greater slowing of response times in the mixed-language condition relative to the blocked-language condition, developmental changes in the pattern of speed-accuracy trade-offs in the mixed condition can be interpreted to reflect a change in the ability to resist cognitive interference during word production.