As by an official announcement or trustworthy reports. The question can resolve anytime until 2030, as long as the source credibly mentions that the neurological disease already was present in the years 2020 - 2024 (including 2024). Early-stage disease and pre-cursors count, such as early-stage Alzheimer's.
Resolves no by the end of 2030 if there's no strong evidence in favor of yes by then.
His 2021 health report doesn't claim any neurological issues, but his stiffening gait and increasing gaffes are worrisome. The health report also does not even touch on cognitive tests properly.
President Biden's Current Health Summary 2021 - The White House https://www.whitehouse.gov/wp-content/uploads/2021/11/President-Biden-Current-Health-Summary-November-2021.pdf
Also, population base rates for Alzheimer's, Parkinson's & co are reasonably high at his age.