Medical student and amateur historian here. Hopefully, I can add some insight.

In pediatrics early developmental milestones are important. A child failing to meet certain milestone dates may need further assessment to determine if there are any underlying issues causing the delays. In many cases these milestones are very specific. However walking is not one of the more specific milestones. According to the American Academy of Pediatrics' Council on Children with Disabilities a child should "walk well" by ~14 months.

The issue with tracking a milestone like this is there is a fairly large range of when a child could possibly start to walk well. It is entirely possible for a healthy child to walk well as early as 12 months or later than 16 months.

There are images in the Hours of Catherine of Cleves manuscript from the 1400's that depict Jesus in a baby walker. Now that doesn't mean Jesus used a walker but the artist probably took their knowledge of the present time period to draw the image. This indicates that baby walkers have been used since the 1400's.

So, with all this being said the specific answer to your question is difficult to answer. Someone would have to take the data from populations of child before and after the invention of baby walker to determine any change in average age. I have not been able to find the data sufficient to determine this. Although 600 years in a historical sense may be a significant amount of time in terms of human evolution this is nothing. A human from 600 years ago would be almost identical genetically. History is not needed to answer this question. Simply, look at two population of modern humans, one population where the family used a walker and one where the family didn't. As u/585AM mention there have been numerous studies showing that certain walkers slow a child's development. Notable arguments were made by Dr. A. Carol Siegel who believes that these walkers may give the appearance that a child is developing but really the child is being hindered but the walker apparatus by not being able to see their own feet.

Just to add this this further, walking is not really a learned trait. A baby left alone with no instruction or assistance would eventually walk. A child is able to walk through the development of their gross motor skills. If I spent every waking second trying to get a 1 month old baby to walk it would never happen because the gross motor function simply are not there.

Yes, giving a child a nurturing home and practice developing their motor skills will help, but the data is simple not available to determine if the average age has changed looking at it from a historical prospective.

I know I kind of rambled on in some places so let me know if anything is unclear.