Age related cognitive decline is extremely well documented. There is no way around it, you can slow it sometimes but everyone shows decline, it's well established. Particularly between 60-80 decline starts becoming rapid.
https://link.springer.com/article/10.1007/s41999-025-01377-8 for example is a longitudinal study of several hundred adults. They find:
Advancing age was associated with poorer performance across all cognitive domains. Processing speed and executive indices declined earliest, with significant decrements evident in the early 70 s. Neither linear regression nor ANCOVA using test year revealed any improvement in neuropsychological test performance; instead, significant deterioration across multiple domains was observed.
If you think people can be too young to think clearly because their brain is far enough from a healthy adult human's as to represent one unsuitable for decision making you necessarily must draw the same conclusions around aged brains. It's not bias, it's just decay. We all rot in the end