Shawn I am totally in your line of thinking. Yes, the 14 y.o.'s have been wrestling with the HS'rs all season but now being a 14 y.o. they can truly compete with their own age group and see what's out there for them as they progress.
I do not like the idea of moving it to grades 9-12. I truly feel you will discourage the numbers of 14 y.o.'s that continue. Kids wrestling has been an option after their wrestling season to match up with the power houses in their age group ...there is a huge difference between a 9th grader and a 12 grader....
Many times I've seen the placers at state not by their own account choose NOT to continue on with Kids' Wrestling after season because they want to move on to another sport or their just tired and need a break since they've been generally playing football and then wrestling and then they want to just relax a few weeks or so.
I really can't see the numbers growing outrageously if you add the 18 year olds....and feel the 14 y.o.'s will be the ones that your "turning off".