Abstract: Purpose: Poor diet quality is one of the key contributors to poor cardiovascular health and associated morbidity and mortality. This study aimed to assess how the short-term associations between diet quality and metabolic risk factors change with age. Methods: This longitudinal, observational study used data from the National Diet and Nutrition Survey (2008–2016) (n = 2024). Diet quality was measured using the Dietary Approaches to Stop Hypertension (DASH) index, fruit and vegetable (F&V) intake, and a F&V biomarker score. We assessed associations between measures of diet quality and a metabolic risk z score (generated from five metabolic risk factors) among those aged 11–60 years, and then tested effect modification by age group (adolescents 11–18 years, young adults 19–35 years, mid-aged adults 36–60 years). Results: Analysis across all age groups showed inverse associations between standardised DASH index and metabolic risk z score of − 0.19 (95% CI − 0.26, − 0.11). These associations were moderated by age group, with strong associations seen in mid-aged adults: − 0.27 (95% CI − 0.39, − 0.16), but associations were significantly attenuated in young adults [− 0.10 (95% CI − 0.22, 0.01)] and adolescents [0.03 (95% CI − 0.05, 0.11)]. Similar results were found for F&V intake and F&V biomarker score. Conclusions: Short-term associations between diet quality and metabolic risk are not consistent across adolescent and young adult age groups, suggesting that mechanisms by which diet impacts on metabolic risk may be acting differently in younger age groups compared to adults. Further research is warranted using longitudinal study designs and replication in different populations to understand changes in determinants of cardiometabolic health with age.