My 11-year-old is running out of storage space in his room. It's too small to accommodate a double, even a small one, as people seem to like. He has a desk (by the window, which I'd rather keep, as he can work by natural light) so a mid-sleeper bed with a desk underneath isn't necessary.
Just a bit concerned that investing in a bed like that will be a false economy if he ends up hating it when he hits his teens. He's currently in an ordinary single bed.