Previous reports have investigated the impact of age on D-Dimer testing in elderly individuals with suspected deep vein thrombosis (DVT), but data on the age-related diagnostic value of D-dimer in a sample covering a broad age range are limited. The present study determined age-specifically the diagnostic accuracy of D-dimer and compared it to C-reactive protein (CRP), a marker of inflammation, in 500 patients with suspected DVT from the VTEval project (NCT02156401). Sensitivity of D-dimer was lower in patients < 60 years in comparison to patients ≥ 60 years (∆-16.8%), whereas specificity was 27.9% higher. Lowest levels of sensitivity were detected for female sex, unprovoked DVT, low thrombotic burden, and distal DVT. A fixed D-dimer threshold of 0.25 mg/L FEU resulted in elevated sensitivity for patients < 60 with a reduction of false negatives by 40.0% for proximal DVT and by 50.0% for distal DVT. In patients < 60 years, D-dimer and CRP demonstrated comparable diagnostic performance for both proximal and distal DVT (p > 0.05). In conclusion, these data outline a clinically-relevant limitation of D-dimer testing among younger patients with suspected DVT indicating a necessity for age-adapted cut-off values. Further research is required to decrypt the role of inflammation in the pathophysiology and diagnosis of venous thrombosis.