Originally Posted by
FBinNY
It's kind of like buying jeans. A parent might buy slightly long jeans for a child to leave room for growth, but at some point we stop growing and can buy jeans that fit perfectly now.
I'm at the same stage as you, I know with good certainty how I'm going to gear any specific bike so I can buy derailleurs the are as exact a match as I can get. However, at this point the OP's bike is still in it's early teens and needs some room for growth. It may not get any taller but the parent (owner) shouldn't bet on that. Too long jeans can be shortened. Too short ones have to be replaced.