If your Birthday's near, ask some relative to get you one for that. That relative may or may not be your parents. If it's not near, do as they say - save up and earn enough to spend it. If you live in a family where one of your parents does not work, your family will be poor, and getting a DS comes out of their money, not yours.
Anyway, by the time you're 12, you should know how to earn. It helps you prepare for a job (in advance).
I have a DS Lite and Wii, and I'm 10 (nearly 11), but I don't get 'pocket money' or an 'allowance', so I rarely get any toys.
DS - two Christmases ago when I was still 9. Wii - last Christmas.
Gamecubes are cheap and fun, or why not get a Game Boy, Game Boy Advance, Gameboy Colour or a normal DS (not lite) as you can save up for that lot in a quick amount of time. I hope I helped...