This longitudinal case study from a Complex Dynamic Systems Theory (CDST) perspective touches upon an under-researched issue: L1 development over the lifespan. Levinson (1978) predicts three stages in adulthood: early, mid and late, with a decline in late adulthood. We examine Diane Larsen-Freeman's publications over a period of 50 years (from age 27 to 77) and trace seven complexity measures--three lexical (density, sophistication and diversity) and four syntactic (mean length of sentence, finite verb ratio, dependent clause per T-unit and complex nominals per clause)--to investigate whether early, middle, and late stages in adulthood occur as predicted. After employing common CDST methods to find out if there are significant peaks or interactions among the variables over time, we used a Hidden Markov time-series analysis to locate moments of self-organization, suggesting a new stage of development. The HMM shows a clear phase shift between middle and late adulthood when the writer was 63. Her vocabulary became more diversified, but her sentences were shorter, but not less complex. Therefore, we argue that this shift should not be seen as a decline in complexity but a shift in style as more precise words may lessen the need for more words.