I don't have links handy, but I remember seeing studies that show brain aging on the order of ~10 years in long covid patients reporting brain fog. That could also be a result of sedentary living as a result of the lockdown, it seems unlikely that it'd be that severe given the length of time involved.

As far as I know the leading theory is that damage to the circulation systems of some covid patients is causing a reduction in the quality of bloodflow in the brain which has obvious negative impacts on neurons.