Introduction: Infections are major contributing factors to morbidity and mortality in the elderly. Aging affects various aspects of antibiotic pharmacokinetics, including absorption, distribution, and elimination. Maintaining adequate antibiotic concentrations is crucial in elderly individuals due to the heightened risk of treatment inadequacy. In this study, our objective was to investigate the plasma concentrations of meropenem, a commonly utilized antibiotic in elderly populations, and assess the impact of age on these measurements. Materials and Methods: In this prospective observational study, we analyzed meropenem levels in a total of 177 blood samples obtained from 59 patients aged 65 and older. These patients were under the care of inpatient services outside the intensive care unit. Meropenem treatment was administered through intermittent infusions of 1 g in 0.5 hours every eight hours. A total of three blood samples were collected from each patient. These samples were collected on the third day of meropenem treatment, just before the next dose, at 30 and 120 minutes after the first dose. Plasma meropenem level was quantified using high-performance liquid chromatography-ultraviolet analysis. To determine the effect of age on the results, the obtained data were compared with the patient characteristics and laboratory parameters. Results: Our results showed that in the first samples (Ctrough) plasma antibiotic concentrations exceeded the MIC in 20.3% of patients, while 79.7% remained at the subtherapeutic level. In the second (Cmax) and third samples (Cmid) 5.1% and 1.7% of patients remained at the subtherapeutic level, respectively. The plasma meropenem level was 8 mg/L and above in participants with four and more comorbidities, and this result demonstrated statistical significance (p< 0.05). Conclusion: Current guidelines for beta-lactam antibiotics do not provide predictable trough antibiotic concentrations in older adults hospitalized for infections. There is a need for predictive factors to inform antibiotic dosing in the elderly population, and a greater emphasis on therapeutic drug monitoring of beta-lactams in these patients would be beneficial.