There is no good answer just yet, but it is either an information theory phenomenon (brain as a prediction machine theory), or this could be linked to activity of dopaminergic, cholinergic systems, or even changes to base neuron firing rates. There is a myriad of possible root causes.

Personally I think the dopaminergic hypothesis is not entirely incorrect [1], and there are treatments for this aspect of ageing.

1. https://www.frontiersin.org/journals/neurology/articles/10.3...