This happens because you try to plot a formula with 3 inputs vs. a single X-Axis.
The table shows that the 2nd and 3rd input is constant.
But Graph Builder doesn't know, because the parameters are not used in Graph Builder.
And this is the solution: Just put the parameters somewhere where they don't hurt (Group X/Y) - and you will get the curve ...
Names Default to Here(1);
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
dt << New Column( "age/height", Formula( :age / :height ));
dt << Graph Builder(
Variables( X( :height ), Y( :"age/height"n ), Group X( :age ) ),
Elements( Formula( X, Y, Legend( 11 ) ) )
)