Together with Teuda and Tkdaya, Truku is one of the highly endangered dialects of Seediq spoken in Eastern Taiwan. Based on respondent narratives produced for a picture-based storytelling task, this paper provides an opportunity to explore the phonological, lexical, morphological, and syntactic differences among individuals in different age groups in speakers of Truku Seediq (hereafter, Truku). These linguistic differences in proficiency in individuals serve as evidence of language attrition of a speech community. The loss of proficiency also indicates language shift, in other words, changes in language use. The purpose of this study is to present a measure of collective proficiency in Truku to enable an assessment of language attrition and create a baseline that can serve as a starting point for developing conservation programs. A total of 72 participants in four age groups (10–15, 16–25, 26–40, and 41–65 years) were tested. All were ethnic Truku. As predicted, all four cohorts exhibit a cline of phonological, lexical, morphological, and syntactic reduction or simplification in language use, suggesting a cross-generational decline of Truku Seediq. Moreover, young adults and youths exhibit a salient reduction in linguistic complexity and tend to produce analytical language structure. This evidence of further decline of proficiency in Truku highlights the urgent need for language maintenance and revitalization.