While I didn't have my 13-year old son do nearly as much math as I had intended him to do this year, what I did is definitely sinking in and has paid off!
He wanted to make a milkshake the other day and found a recipe online to do so. But he saw it would make 4 servings and he only need one. The only help I offered was he would have to divide everything by 4. He easily did the division in his head, even 2 divided by 4 being one half.
I have hope for this child yet! lol (Ok, he's really not that bad. This is so typically him: leave me thinking nothing's sticking and then effortlessly does something.)