There are multiple definitions:

1. American Academy of Ophthalmology(English)

   • Ophthalmology, Organizations (scientific)

2. age at onset(English)

   • young-onset PD patients show a good response to levodopa, more dyskinesia, dystonia, and slower progression, while late-onset PD patients demonstrate more axial symptoms, cognitive impairment, and genitourinary symptoms
   • PD
   • Family physician, Prognosis, Parkinson, Cognitive function
   • https://doi.org…598-020-68301-x