This longitudinal study with time-serial data examines for the first time whether different types of intraindividual variation in second language (L2) performance and cognitive functioning are related, and how and when they influence L2 development longitudinally in older adulthood. We analyzed the L2 development of 26 German-speaking adults aged 62-79 who were taught L2 English for 2 × 90 minutes per week over 6 months. At each of the 15 measurements, the participants completed three L2 tasks and eight cognitive measures, and they answered open-ended questions about socioaffective variables such as L2 motivation. Results of generalized additive mixed models and qualitative content analyses showed, inter alia, that L2 variability--rather than inconsistency or dispersion--had a (nonlinear) effect on L2 growth, being especially large during periods of rapid development. The qualitative analyses revealed a blended operation of internal and external states being associated with periods of significant L2 growth.