Cognitive decline is a hallmark of brain ageing. Leukocyte telomere length (LTL) has emerged as a candidate biomarker related to brain ageing and neurodegeneration; however, reported associations with cognition and brain structure vary across cohorts. Long‑chain omega‑3 polyunsaturated fatty acids (PUFAs), notably docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), exert anti‑inflammatory and antioxidant effects that may, in some contexts, relate to slower telomere attrition. Here, we synthesize evidence on n‑3 PUFAs, telomere biology, and cognitive outcomes, integrating clinical, epidemiologic, and experimental data. We emphasize biological plausibility (oxidative stress/inflammation, membrane remodeling, mitochondrial function and expression of telomerase reverse transcriptase (TERT) through PI3K/Akt/mTOR, NRF2 and epigenetic modifications) while acknowledging heterogeneous human findings and methodological considerations (assay variability, life‑course timing, cognitive domains, and biomarker stratification). We outline priorities for future studies to clarify causal pathways and inform dietary strategies that support healthy cognitive ageing.