Good Gawd, Jim, why on Earth wouldn't previous laptops be able to run 10.5? Of course they can.
If you order a new laptop it will most likely ship with 10.5 preinstalled. If you buy one that's been on the shelf then it'll probably come with Tiger on it and a free box of 10.5.
Question is, will your car run on gasoline refined from $100/barrel oil?