to say on average that atheists are smarter than believers is the same as saying on average religious people are dumber than atheists. the data does seem to support that religious people have statistically significant less iq points, on average…
but individuals obviously vary for all kinds of reasons. theres plenty of very smart believers. my wife, for instance is very smart, and was a very devout catholic for many years. lots of reasons. tradition. strong family indoctrination. devastating losses int he family. ritual comforts. others…i would never mock anyone personally for believing. on the other hand, some believers in my exerience have no problem pitying nonbelievers, which to me feels like a form of mockery.
i do think it plausible that smart people ar emore likey to lose the faith with age. need data on that.