In this study, the effects of second language (i.e., L2) proficiency and age of second language acquisition are assessed. Three types of bilinguals are compared: Early L2 learners, Late highly proficient L2 learners, and Late less proficient L2 learners. A lexical decision priming paradigm is used in which the critical trials consist of first language (i.e., L1) Spanish targets preceded by English (L2) primes. Three types of L2-L1 priming conditions were used: Semantic primes [e.g., PRIME=nail; TARGET=TORNILLO ("screw")], Mediated Form primes [e.g., PRIME=bull ("toro"); TARGET="TOR"NILLO], and Form primes (e.g., PRIME="torture"; target="TOR"NILLO). Semantic and Mediated Form primes produced facilitative priming effects for the Early group, but not for either of the Late groups. Highly proficient Late learners showed inhibitory effects of Form primes, whereas the less proficient group produced no priming effects of any type. The pattern of priming effects is consistent with an interactive activation architecture in which the semantic/conceptual level is shared by L1 and L2 for Early learners, versus a shared lexical level for proficient Late learners. The implication is that age of L2 acquisition has a major influence on how bilinguals represent and access words in their second language.