In my experience there is always a mixture:
Some were not so sharp, and others were very intelligent. So I dont think its so much a matter of intelligence in all cases. Even the mind of an intelligent person is capable of performing some amazing mental gymnastics in order to accomodate a desired worldview.
There are some elders who have so much invested in years, family and effort, that it would be mental suicide to allow themselves to allow that it all was a waste and a lie. They may be very intelligent, and can reason step-by-step in a line of reasoning, but as soon as there belief system is threatened from the outside or by their own thoughts the "dumb down" switch goes on. It almost a self-preservation mode. These are the ones who are the most frustrating to deal with, as it is confounding and inexplicable how intelligent people can follow a line of reasoning so well and then shift course in reasoning so abrubtly.
Others are in it for social and family reasons and fear of the outside unknown holds them in. They may be elders but are still using training wheels when dealing with anybody not a jw. These are the ones who were raised in and have grown up in the jw cocoon.
Others delude themselves into thinking they have real power. They probably are ineffectual at getting power in other areas (i.e at the job) and so use the org as an outlet to wield some untapped desire for power. And then there are others who are the ladder climbers. They like the power and enjoy the attention received from being front and center at assemblies, giving talks, and being of the prominent ones in the org and well as in their careers.
The true believer elders are among those who are more simple-minded and never really have a thought apart from what is given to them. The one issue they all have in common is dealing with reality... some choose not to ...others may not have the ability to. Some really do need the jw environment.
The elders who are not of the above, but willing to question and can reason, usually don't remain elders.