There is good evidence for the benefit of green tea consumption for brain health, of course, in the context of a healthy dietary pattern. For more reading, here are some good quality references:

Diabetes and Alzheimer’s disease: can tea phytochemicals play a role in prevention? Journal of Alzheimer's Disease 59.2 (2017): 481501.
PMID: 28582855

Green Tea Intake and Risks for Dementia, Alzheimer's Disease, Mild Cognitive Impairment, and Cognitive Impairment: A Systematic Review. Nutrients, 2019 May 24;11(5):1165.
PMID: 31137655