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Background: Specific cell types from people with Down syndrome (DS) show faster accumulation of DNA damage and epigenetic senescence marks. Causative mechanisms remain un-proven ranging from amplified chromosomal instability to actions of chromosome 21 genes. Plasma immunoglobulin G (IgG) glycosylation profiles are established as a reliable predictor of biological and chronological ageing. Methods: We performed IgG glycan profiling of n=246 individuals with DS (208 adults and 38 children) clinically characterised for co- morbidities, from three European countries, and compared these to age-, sex- and demography- matched general populations. Results: We uncovered very significantly increased IgG glycosylation ageing marks associated with DS, beginning in early childhood. Average levels of IgG glycans without galactose (G0) and those with two galactoses (G2) as a function of age in persons with DS corresponded to levels detected in 19 years older euploid individuals. Sub-cohorts of DS without any known co-morbidities from different countries showed a similar result, eliminating DS- linked autoimmune diseases, frequent infections, or Alzheimer’s dementia as indirect causes of accelerated biological ageing. Importantly, no difference in slope of the curve relating IgG- glycan-age to chronological age was observed for any of the 3 analysed populations, discouraging the amplified-chromosomal-instability explanation. Remarkably, IgG-glycan profiles of a child with DS caused by a short segmental duplication of only 31 genes on chromosome-21 (DS-critical-region) had values like age-matched whole-chromosome-trisomy- 21 children, pointing to a causative contribution of one or more of the 31 genes. Conclusion: This shifts the paradigm from an (essentially untreateable) “amplified instability” mechanism, to a specific gene-overdose-driven cause of a progeria-like syndrome, opening possibilities for therapeutic amelioration strategies. We generated an induced pluripotent stem cell (iPSC) model of this child with partial trisomy 21, and editing experiments using CRISPR-Cas9 are underway in order to dissect the causative overdosed genes and mechanisms responsible for the accelerated ageing in DS.