After initial benefit, many adverse effects, i.e., 'Wearing-off' reactions, 'on-off' fluctuations, dyskinesias or therapy. Patients with Juvenile Parkinson's disease develop response fluctations and dyskinesias earlier and with a high incidence than those with old onset Parkinson's disease (OOPD). In order to elucidate the most probable factors for this wearing-off phenomenon and dyskinesias in relation to the age at onset of idiopathic Parkinson's disease (PD), we investigated several parameters such as the duration of the illness and of the Levodopa therapy in 252 patients under Levodopa treatment. We also evaluated the characteristics of initial symptom in patients with different age at onset. We identified two groups of patients: The first consisted of young onset PD cases (YOPD) with onset before age 40 and the second of OOPD cases with onset after age 41 and which was subdivided in two subgroups. The further consisted of cases with onset between age 41 to 55 (middle :age onset PD (MOPD)) and the latter consisted of cases with onset after age 56 (Later onset PD (LOPD)). Our results suggested that the initial symptom was bradykinesia in the YOPD group and tremor in the OOPD group. The numbers of patients with YOPD who developed wearing-off and dystonia were found higher than those of patients with onset after age 41, (p< 0.001). In considering the duration of the illness, all the side effects appeared later in the MOPD group than LOPD group and these results were statistically significant (p< 0.001 for dyskinesia and dystonia, p< 0.01 for wearing-off phenomenon). When considering the duration of Levodopa therapy, it was showed that patients with MOPD developed side effects much more later than those with OOPD and these results were statistically significant (p< 0.02 for dyskinesia and p< 0.01 for dystonia). In the YOPD group, the dyskinesia developed later than dystonia and wearing-off phenomenon (mean 4.3 ± 3 years). The localization of dystonia and dyskinesia was not statistically relevant in the different groups.