that is the most ridiculas analogy ever>
A 6`2 17 y/o is not the same as a 6`2 13 year you have to figure in muscle growth.
When would you like the age disparity to stop 1 year 2 3 why not go too- young for your grade- and double hold back. Do you think that is fair.
Look at Pop warner study where that have strict age cut off- team that is (Oldest) closest to age cut off wins 80% of time.
We are not taking years here just months.

It is a enormous advantage to hold your son back. It has to be regulated or it will just get out of control