Purpose: Clinical grammar markers are needed for children with SLI older than 8 years. This study followed children who were previously studied on sentences with omitted finiteness to determine if affected children continue to perform at low levels and to examine possible predictors of low performance. This is the first longitudinal report of grammaticality judgments of questions. Method: Three groups of children participated: 20 SLI, 20 age controls, and 18 language-matched controls, followed from ages 6-15 years. An experimental grammaticality judgment task was administered with BE copula/auxiliary and DO auxiliary in wh- and yes/no questions for 9 times of measurement. Predictors were indices of vocabulary, nonverbal intelligence, and maternal education. Results: Growth curve analyses show that the affected group performed below the younger controls at each time of measurement, for each variable. Growth analyses show linear and quadratic effects for both groups across variables, with the exception of BE acquisition, which was flat for both groups. The control children reached ceiling levels; the affected children reached a lower asymptote. Conclusion: The results suggest an ongoing maturational lag in finiteness marking for affected children with promise as a clinical marker for language impairment in school-aged and adolescent children and probably adults as well.