Mainstream linguistics has long held that there is a fundamental difference between adult and child language learning (Bley-Vroman, 1990; Johnson & Newport, 1989; DeKeyser, 2000; Paradis, 2004). This difference is often framed as a change from implicit language learning in childhood to explicit language learning in adulthood, which is presumably caused by maturation. However, the position that children learn implicitly and adults learn explicitly relies on studies done only with adults (e.g. R. Ellis, 2005; Norris & Ortega, 2001; Spada & Tomita, 2010). No research to date has compared child second language (L2) learners on tasks tapping implicit vs. explicit knowledge separately. Moreover, the position that this change is caused by cognitive maturation ignores the fact that adult language learners typically receive more explicit instruction than child language learners (Nikolov, 2009). Based on literature to date, we do not know whether child L2 learners do, in fact, learn implicitly, and if so, whether they do so because they are children, or because they typically receive no explicit instruction. This dissertation comprises two empirical studies that tease apart the effects of age from the effects of instruction on implicit vs. explicit second language learning. The first study compares the performance of elementary and high school classroom learners of Spanish, who receive different types of instruction, on a story listening-and-rewriting task, which taps implicit knowledge, and a verb conjugation task, which taps explicit knowledge. The second study manipulates instruction, teaching child and adult participants an artificial mini-language under controlled implicit or explicit training conditions. Major findings of Study 1 are that (1) child learners who also receive implicit instruction do favor implicit knowledge, (2) adolescent learners favor implicit knowledge if they receive implicit instruction, but explicit knowledge if they receive explicit instruction, and (3) a small amount of explicit instruction quickly changes patterns of performance. Study 2 finds that (1) both children and adults develop greater awareness of grammatical rules under explicit training conditions than implicit training conditions, (2) both children and adults produce an artificial mini-language more accurately when their attention is directed to form, and (3) adults, but not children, may "spontaneously" develop explicit knowledge even under implicit training conditions, but this may be related to their prior foreign language instruction. Based on these findings, I argue that the change from implicit language learning in childhood to explicit language learning in adulthood is not caused by age alone. Instead, instruction has a significant influence on implicit vs. explicit learning, at any age. This contradicts the strong version of the critical period hypothesis for implicit learning (DeKeyser & Larson-Hall, 2005). Theoretically, these results support views of child and adult second language learning as qualitatively similar. Pedagogically, results suggest that child and adult L2 learners can take advantage of both implicit and explicit learning capacities. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]