You are only going to live a certain number of years.
There is a finite point (at least in the U.S.) where you have the freedom to do what you want. I put that point as high school graduation. I do personally know some people that broke this mold, said f*** it to the rules, took the GED and bailed out of high school at 15 or 16, and they are highly successful folks.
But for most of us it is high school graduation. At that point, you are going to make some decisions for yourself. Finally. You have reached the end of the scripted presentation. I could feel the chains falling off. Leaving home and being on my own for the first time was the greatest feeling I have ever had. The tutorial is over, it's time to start the game.
I submit the following letter, from your 4 or 5 year old child, unable to articulate his or her future self's wishes:
Dear mom and dad:
Today you are considering holding me back one year, because you are afraid. You are afraid that I may not be ready to start this journey. You are afraid that over the next 13 years I will be too small, or too awkward, to successfully navigate school.
Do not let your fear rob me of a full year of freedom. Right now I am a slave to my own dependence on you. When I graduate high school, I will be free. At that time, my decisions, in a very real sense, will overwhelmingly dictate my successes. I only have so many years of freedom before I die, and this right here, this is your chance to give me an extra one.
It might not work out, I might not develop fast enough, they may have to hold me back somewhere along the way. I may not make the varsity team of a sport I wouldn't be able to play in college anyway. I may lack the emotional development to start dating in seventh grade. There may be an emotionally stunted coach who says I'm too short to play basketball, or any other of a hundred problems. But there will be problems regardless, and avoidance of them is not nearly as valuable as what can be learned from dealing with them.
Please have faith in yourself as a parent, to deal with these things as they arise, together with me. I will struggle from time to time. It may be harder. But ease is not the point.
What will I do over the next year that will be better? If you hold me back, you have to make it worth it. What language will we learn? What physical training will we do? What fantastic experiences have you lined up for me in the next 12 months? Because in 13 years, when I'm still one year away from freedom, because of this decision, there will be a reckoning.
And if I'm not in the top 10. And I'm not athletic-scholarship-bound. Which is super likely. Then it will be a lost year.
-Your kid.
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Anectodotally, I could have started a year earlier, and I resent this decision forced on me by others. Kids in my grade offered no challenge, school was mind-numbingly boring. And I was never going to excel at sports because I don't particularly care for competition. I took as much summer school as was available and ended up with a senior year with only 3 classes. Stuff that was sequential that you just had to have in order to graduate, because rules.
It took me 14 months to earn a master's degree. So holding me back 12 months, we could roughly compare to the career implications of an advanced degree.
Your future doctor will be a doctor for one extra year if you don't hold her back.
Your future supreme court justice will be one for an extra year if you don't hold her back.
Your future NFL quarterback (or coach!) will be one for an extra year if you don't hold him back.
Your future NFL cheerleader won't be as old and wrinkly if you don't hold her back!
And your future average american professional will have an entire extra year to earn money or get an advanced degree if you don't hold them back.
K-12 IS NOT DIFFICULT. You don't know what is going to happen.
Be not afraid.