Purpose: Previous studies documenting longitudinal linguistic outcomes of children with specific language impairment (SLI) compared to their age peers focus on the property of obligatory finiteness marking in sentences across the age span of 5-18 years. This study evaluates tag questions as syntactically complex sentences that extend the demands of finiteness marking across clauses, requiring coordination of negation in the base sentence and tag question. Method: Five hundred eleven children (240 unaffected, 271 SLI affected), between 5 and 18 years of age participated, following a rolling recruitment longitudinal design, which included a total of 4,718 observations. The linguistic task was designed to evaluate four variations of tag questions, two of which targeted polarity requirements for tags and two of which were nonpolarity differences in the tag. Growth modeling methods were used to test hypotheses of group differences (SLI vs. unaffected) in understanding of tag questions over 5-18 years. Covariates were child nonverbal IQ, mother's education, and child sex. Results: Children with SLI's outcomes varied by age and item type. They performed below unaffected children across all tag outcomes at 10 years, scored correctly on nonpolarity items at 18 years (ceiling levels), and continued to lag unaffected children at 18 years on polarity items. Significant SLI effects on the outcomes were not moderated by the covariates. Conclusions: By 18 years, the SLI group performed the nonpolarity items correctly but continued to struggle with polarity items. Thus, polarity is of interest as a possible screener for SLI throughout the school years.