How do child and adult L2 classroom learners integrate newly-learned L2 words into their L1 mental lexicon, and how do they process the orthographic, phonological, and semantic codes of words in their L2 and L1? Do these processes differ in child L2 learners, who are still developing their L1 lexical knowledge, from adult L2 learners, and if so, how? Current psycholinguistic models of lexical processing in L2 learners, the Revised Hierarchical Model (RHM) and the developmental specification of the Bilingual Interactive Activation model (BIA-d), focus on adult L2 learners. A guiding question in this chapter is whether these models can also account for lexical processing in child L2 learners. The chapter focuses on two critical assumptions these models share: (1) lexical access is language nonselective, and (2) L2 word meaning access progresses from reliance on lexical-level links between L2 and L1 to reliance on direct L2 word form-to-concept mappings. Psycholinguistic studies testing adult and child L2 learners, with a specific focus on classroom learners tested during the earlier stages of L2 learning, are reviewed. The currently available evidence indicates that lexical processing in child and adult L2 learners shows remarkable similarities, despite considerable developmental differences in language and literacy knowledge, brain maturation, cognitive function, and processing speed. Both child and adult L2 learners' lexical processing support language non-selective and parallel activation of L1 and L2 orthographic-phonological codes. However, child L2 learners seem to exploit L2 word-to-concept links at an earlier stage in L2 learning than adult L2 learners, and L2 meaning access appears less reliant on the L1 system in children than in adults.