This thesis examines the acquisition of finiteness in English by child L2 learners by investigating the impact of the age of onset and the role of the learners’ L1 on their L2 acquisition. Following Meisel’s hypothesis that children older than 4 will resemble adult L2 acquisition in the domain of inflectional morphology, I investigated how two groups of children of different L1s and older than 4 learn the features of tense and agreement and whether accuracy would be declining as an effect of an older age of onset. Participants were 73 Chinese and 74 Russian learners who were aged either 9 or 12 at time of testing and had age of onset of learning English at ages 4 and 7 respectively. Children were all EFL learners recruited from EF (English First) private afternoon English language schools in Shanghai and Moscow, where children attended classes for a few hours a week. To assess children’s performance, I employed two types of tasks: two elicited production tasks whose prompts involved 3SG-agreement and past tense contexts (TEGI), and a freer type of elicitation prompting stories based on a sequence of pictures (MAIN). Data analysis demonstrated low accuracy, high numbers of omissions, asymmetries in the acquisition of morphemes, overgeneralisation of the progressive tense in 3SG-habitual contexts, and use of the periphrastic structure ‘is + verb(x)’. These results show that L2 children resemble aL2 acquisition supporting Meisel’s hypothesis. The empirical findings are interpreted in light of two opposing views that account for the optionality in verb inflection in L2 acquisition; the Full Access to UG and the Representational Deficit approaches; as argued data are more consistent with a representational deficit account. Older children consistently outperformed younger ones; as features are inaccessible, older learners compensate by relying on their higher cognitive abilities, learning strategies and metalinguistic skills, while younger children are mostly implicit learners using more the periphrastic structure as immersed children do. The periphrastic structure appears to be a stage in L2 development of verb morphology in English which denotes the emergence of finiteness as a category being triggered semantically through interpretable features of be. This is a first stage toward activation of uninterpretable features. Finally, signs of L1 influence became more pronounced in older learners; it was the older children showing more L1 effects, a finding which is again more consistent with a representational deficit account.