Is there a name for the assumption that people in past ages were less able to reason?

CS Lewis and others have called this "Chronological Snobbery". See Wikipedia on Chronological snobbery.

...what I have called my "chronological snobbery," the uncritical acceptance of the intellectual climate common to our own age and the assumption that whatever has gone out of date is on that account discredited. -CS Lewis

This is simply an example of temporal chauvinism:

chauvinism, noun: undue partiality or attachment to a group or place to which one belongs or has belonged [MW]

Edit: Jay and Peter Shor suggested that temporal chauvinism is the specific term that captures this attitude. While Googling temporal chauvinism, I came across this paper that contains a paragraph that could have been written with this question in mind, and introduces another word to express the concept: chronocentricity.

Like ethnocenricity, the word chronocentricity is meant to convey an unconscious bias. The bias, however, is related to time (chronos) and not ethnicity. Chronocentricity is a neologism that you won’t find in the dictionary. The closest comparable terms I’ve come across are “temporal chauvinism” and “generational chauvinism.” The underlying idea is that we mistakenly think of our own generation as better than any other generation. We think that we represent the highest point of world civilization. We look back to earlier times and cluck our tongues at how benighted, “quaint,” and misinformed earlier generations tended to be. We uncritically assume that our quality of life is better, and that people “back then” just didn’t know enough, or have the right kind of technology, to live properly and adequately.

Sounds for the most part like the appeal to novelty logical fallacy, or argumentum ad novitatem.

However, there's a fine distinction to consider. The appeal to novelty represents a prejudice in favor of the recent or the new. "It is more modern, ergo it is superior."

What you describe is a complementary prejudice against the past: "It is old, ergo it is inferior."

I also considered chronocentrism, but that again does not quite capture your full meaning. That denotes the assumption that one's own times are the most important and relevant in history.

Again, finely interpreted it is a bias in favor of the present, not against the past.

When speaking of people, ageism would be appropriate, but when speaking of ideas it is not correct.

I'm still pondering a proper fit. Perhaps these suggestions will steer one of us in the right direction.

I'm unaware of any single-word term for the assumption you refer to, but it appears that the phrases myth of primitive societies [1] and myth of primitive society [2] are used by some anthropologists to refer to it. In [1], the first paragraph in part remarks

Humans who live in technologically primitive societies aren’t as intelligent as those who live in advanced societies, or so some of history's most influential personalities thought. The reasoning behind their thinking ... is that primitive societies just harvest what is naturally provided, so ... there isn’t much innovation in the techniques used to collect and store it, and because there is no innovation the brain doesn’t develop all the skills ...

The discussion in [2] is diffuse, but after mention of some of the same issues as noted above, on page 10 appears

Not to put too fine a point upon it, the history of the theory of primitive society is the history of an illusion ... its basic assumptions were directly contradicted by ethnographic evidence and by the logic of evolutionary theory itself.