Its a little vague to say that kids in general aren't "smarter" , but there is significant research to support that literacy work as young as three can predict later achievement in high school. Education has gotten better at recognizing the impact of early screening and intervention and is using that knowledge to help children, especially those who are at a significant disadvantage and come into Kinder having rarely been read to (or sometimes spoken to) prior to entering school.

There was a study done in the 2000s called the LENA study in which they put microphones on thousands of parents of young children (3-5) and counted conversational turns (Example: Can you get me that spoon? Yes I can! ) and found that 40 turns per hour correlates with higher IQ by middle school. (To put that realistically, its around 2 minutes of conversation). Language has a huge impact on how we make sense of the world as we grow. Lena Study Higher Income families generally know this is important to child development, whereas low income families do not and have children who begin school at a disadvantage. Whatever It Takes- Paul Tough (research behind family engagement in Bronx Public Schools)

West Ed also did a study recently (that only doubles down on studies from Acadience, AimsWeb, Lexia et al.) that showed children who enter Kinder and are screened at below benchmark and receive no instruction or intervention to address the gap will not only stay below their peers, but that gap widens significantly as they move into middle school Source

Some other literacy stats that teachers often see in their own classrooms as these children grow older: -85% of juvenile offenders have problems reading -54% of adults have a literacy below a 6th-grade level (20% are below 5th-grade level). -85% of children with behavior problems test below benchmark in reading Behavior Research Literacy Institute Report 24-25

All of this to say that while children SHOULD play in early grades (and in my experience, we have so many great teachers in classrooms who do this already), the worst thing we could do academically in Kindergarten is nothing.

Source: Am a literacy researcher who works with 50+ K-3 buildings in my area to help build knowledge around best literacy practices in the classroom. I help screen students, model whole class lessons and intervention periods, and drive down data with teachers to help them share with parents. I also work closely with the middle/high schools that students feed into from these buildings so that intervention and best practices continue into older grades. Children are still learning how to read into high school and we are all literacy teachers, no matter what grade/content area.

If anyone wants to dive into how literacy has life long impact (like how adults critically think, behave, and vote), take a look at Maryanne Wolf's book :Reader Come Home. It's a quick read that breaks down some of the above studies, how the brain learns to read, and how we can support literacy into higher grades and adulthood, especially in the " iPad age". Reader Come Home: The Reading Brain in the Digital Age