Background: Aging implies a higher prevalence of chronic pathologies and a corresponding increase in medication. The correct adherence and use of the medication are prerequisites for reducing risks of disease progression, comorbidity, and mortality. Medication literacy (ML) is the specific ability to safely access and understand the information available concerning medication, and to act accordingly. Currently, there are few specific instruments that ascertain the extent of ML in the general population. The aim of this work was to analyse ML in a large cohort of pharmacy customers. Methods: A total of 400 community pharmacy clients were analyzed to assess the level of ML (documental and numeracy) through the validated MedLitRxSE tool. Results: The results showed that out of a total of 400 community pharmacy clients only 136 (34%) had an adequate degree of ML, while the rest of the clients (n = 264; 66%) were adjudged not to have this ability. Statistically significant differences were found between the different age groups in terms of ML (P < 0.001; OR = 0.312; 95% CI: 0.195–0.499), the 51–65 and >65-year age groups having a lower frequency of adequate ML (23.5 and 7.1%, respectively) than the rest of the age groups. A statistically significant increase in adequate ML was observed as the academic level of the clients increased (P < 0.001; OR = 15.403; 95% CI: 8.109–29.257). Multivariate logistic regression confirmed the influence of both variables on ML. Conclusions: An inadequate ML level was found in community pharmacy clients over the age of 51, and also in those with primary or non-formal studies. Our data add to our knowledge about ML, and should pharmacists and other health professionals to adopt new strategies to prevent, or at least reduce, errors in taking medicines, thus avoiding the undesirable effects of any misuse.