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Background: Individuals with Down syndrome (DS) have more difficulties with the structural aspects of language, including morphology (concatenation and non-concatenation) and syntax (word order and grammatical/concord rules), than with other language components (e.g. vocabulary, phonetics and pragmatics). Objectives: This study investigates the accuracy of grammatical number inflection produced by Jordanian Arabic-speaking children and young adults with DS. The work also examines the correlation between age and the correct production of singular, dual and plural numbers. Methods: The study involved 60 monolingual Arabic children and young adults with DS, 30 males and 30 females, enrolled at the Nazik Al Hariri Welfare Centre for Special Education, Amman. The participants were divided into three groups: KG2 (7.1–12.5 years old), school (13.10–17.6) and vocational training (18.3–27.3). The participants’ data were collected from a picture elicitation task and free speech, and the answers were recorded using a smartphone. Tokens were classified into correctly used, incorrectly used or not recognised. Proficiency percentage in using the correct number in correlation with age was calculated adopting Jia’s (2003) composite score of proficiency. The one-way analysis of variance was used to trace the impact of age on the correct performance of number. Post hoc comparisons (guided by the Scheffe test) were calculated for the cumulative results of the scale as a whole to examine the difference in the arithmetic mean between the three groups. Results: The singular form was the most used by all age groups (83.3%), followed by the plural (27%); the most delayed was dual (10.3%). Intriguingly, the dual form is the most difficult plural pattern because it was the least frequently used pattern in everyday language. Results were in line with other research on morphological markers in individuals with DS (e.g. Penke, 2018). The cumulative results statistically prove the influence of age on the correct use of grammatical number, in favour of the older two groups (total F = 29.865, at the level of significance P = 0.000), with a higher arithmetic mean of all categories (AM: KG2 = 9.00, school = 15.10, VT = 16.25). Hence, sensitivity to the correct number option increases with age although children and young adults with DS do not reach adult-like performance. The non-recognition cases of the proper number category significantly mark language delay in participants with DS. Conclusion: The study concluded that inflection for grammatical number is evidently delayed in individuals with DS. Linguistic teaching and training of children with DS (involving families, caregivers and educators) should start from childhood and continue to adulthood to improve their use of dual and plural numbers.