https://www.wsj.com/health/wellness/...?mod=wknd_pos1

WSJ article (paywall) reporting on research that the brain ages in five phases...

0-9 Youth
9-32 Adolescent
32-66 Stable
66-83 Slight Decline
83+ More Decline

The last two are characterized by less cognitive flexibility, while remaining more functional in frequently used pathways and processes, ie habits and routines.