Infected elderly people often present with signs and symptoms that differ from those in younger adults, but data on the association between patient age and presentation of early Lyme borreliosis (LB) are limited. In this study, the association between patient age (18–44 years, young vs. 45–64 years, middle-aged vs. ≥ 65 years, elderly) and disease course, microbiologic characteristics, and the long-term outcome of treatment was investigated prospectively in 1220 adult patients with early LB manifesting as erythema migrans (EM) at a single-center university hospital. Patients were assessed at enrolment and followed-up for 12 months. Age was associated with comorbidities, previous LB, presenting with multiple EM, and seropositivity to borreliae at enrolment. The time to resolution of EM after starting antibiotic treatment was longer in older patients. At 12 months, 59/989 (6.0%) patients showed incomplete response. The odds for incomplete response decreased with time from enrolment (odds ratio (OR) of 0.49, 0.50, and 0.48 for 2-month vs. 14-days, 6-month vs. 2-month, and 12-month vs. 6-month follow-up visits, respectively), but were higher with advancing age (OR 1.57 for middle-aged vs. young, and 1.95 for elderly vs. young), in women (OR 1.41, 95% confidence interval (CI) 1.01–1.96), in patients who reported LB-associated constitutional symptoms at enrolment (OR 7.69, 95% CI 5.39–10.97), and in those who presented with disseminated disease (OR 1.65, 95% CI 1.09–2.51). The long-term outcome of EM was excellent in patients of all age groups. However, older patients had slower resolution of EM and higher odds for an unfavorable outcome of treatment (OR 1.57, 95% CI 1.05–2.34 for middle-aged vs. young; and OR 1.95, 95% CI 1.14–3.32 for elderly vs. young), manifested predominantly as post-LB symptoms. The presence of LB-associated constitutional symptoms at enrolment was the strongest predictor of incomplete response.