Here's a photo of my family on a trip to Florida. Something that is totally unremarkable for a family photo is that everyone is different ages. Of course people who are different ages play together. The kids who want to swim go in the ocean. The kids who want to build sand castles go find shovels. No one says, "Ten and eleven year olds all go together!" Because who cares how old the kids are?

In the classroom it's totally different. It's organized by age instead of interest or ability. But once we start examining this system closely, we find many problems with grouping kids by age.

1. Grouping kids by age instead of readiness means kids don't learn to read.
Lisa Nielsen, my friend who oversees how NYC public schools use technology, has something to say on this topic. Of course. She points out that "there is nothing scientific that says we are all good at the same thing at the same time." In fact, child development research shows kids learn in age ranges, and that the range for learning to read is much wider than what is taught in schools. Nielsen says grouping kids by age is one cause of so many kids failing to learn to read – they were not ready when it was taught.

2. Grouping kids by age is a huge detriment to the younger kids in the class. 
The most obvious problem is that kids do not all learn at the same pace. The New York Times reports that there are other problems as well because the youngest kids are at a huge disadvantage. University of Iceland epidemiologist Helga Zoega shows that until seventh grade, the gap in math test scores was largest between the oldest and youngest kids in the grade. Children in the youngest third of the class were 90 percent more likely than the oldest third to score near the bottom on math.

3. Grouping kids by age encourages medicating younger kids so they keep up.
Zoega also finds that the youngest kids in the class are fifty percent more likely to receive medication for ADHD than the oldest kids and concludes that in many cases, "the youngest kids in the class are just acting according to their age. But their behavior is thought of as symptoms of something else."

4. Inappropriate age grouping for young kids has lasting effects.
The achievement gap that starts in the younger grades has lasting impact: Malcolm Gladwell reports that professional hockey players are inordinately from the oldest third of their grade—they perform better throughout their life as athletes, being significantly larger than the other kids they compete against. And The Week reports previous studies have shown that the youngest kids in a grade are ten percent less likely to enroll in college.

5. The problem grows as smart parents ensure their kid is oldest.
Parents and educators in private schools are recognizing the problem for the youngest kids, especially the boys. It's so common for parents to hold back boys with late birthdays that it's become the norm. Who wouldn't do that if they could? The cost is merely one more year of child care before school, for parents already footing the bill for private school,  and it give the kids a huge advantage.

So it is not controversial that kids who are older for their grade do better in school. What is controversial is what to do about the problem. We can hold kids back, but it's a zero-sum game. If you hold all the kids back one year, then that's not holding them back. That's just doing what's normal, and they'll still end up in the younger half of the class.

The solution, of course, is to stop grouping kids by age. Of course it's the extremely expensive schools that group kids by interest and ability instead of by age. For one thing, you would not get federal funding. So it's too costly for most schools to operate this way. And it's another reason why you are better off homeschooling your kids.

Ironically, one of the big arguments against homeschooling is that kids do not get the social exposure at home that they do in school. But it turns out that school creates an artificial pecking order based on what month your child is born. That's not socialization. That's a pre-mature tracking system. The best way to socialize your kids is to let them play and learn with whoever is right for them at that time. But you already knew that. Because that's how your extended family works, too.