Children are unique, so are their milestones
Disclaimer - I am still learning about the developmental milestones changes made by the CDC and AAP and what evidence lead to that decision. (And wondering why PTs, OTs, and SLPs were not consulted.) Although I believe the CDC and AAP had the best of intentions, the final results are receiving a lot of push back from many who work with and advocate for young children.
The following is based solely on my professional opinion for what it is worth.
My take on milestones has always been they are not precise timelines that children have to meet at the exact ages listed. Rather, they are approximations to give an idea of when a child usually meets them. There should be fluidity when approaching them and an understanding there is a variance of time around the listed ages and stages. Children are unique, so are their milestones. I have been vocal about this for years and believe the pediatricians and specialists I used for my children took the same stance.
Before I go further, I want to state, I firmly believe guidelines such as these should be revisited to ensure they align with our current knowledge and research. I also believe information is ever evolving and should reflect the cumulative data received thus far ensuring it is deemed valid and reliable. Additionally, I think it important to consult other disciplines that relate to the field but offer a different perspective.
Are we seeing delays in what was once considered "typical" child development? Yes - we have seen this steady decline the past couple of decades. Is this due to actual changes in child/ human development, or a transformation in our lifestyles? My professional belief is our lifestyles have contributed to this.
Of course, children are talking later. As adults, we get sucked into our screens and talk less with children reducing the amount of language and words they hear. We plug children into devices instead of letting them experience authentic sounds in their environment. This is not conducive to language development. Yes, children are walking later. We have created holding devices to restrain children either in the name of safety or for our convenience and provide environments that in no way encourage free movement. Children cannot develop physically in those situations. And much like walking, children crawl later due to being contained too much. Removing crawling totally as a milestone is something I completely disagree with as it is critical to physical and brain development.
One more thing I would be amiss not to mention, as a society we have robbed children of their right to play which is vital to child development and we are seeing the adverse effects now.
Does all of this constitute changing developmental milestones? OR should the focus be on adapting our lifestyles to support child development? For what it is worth, my vote is for a lifestyle change.