I would say the cutoff is Game Boy Advance SP and Micro is a poseur, because Micro can't play original Game Boy or Color games. If one is counting GBA, then the DS and DS Lite could still play those.

Edit: Get a flash cart and you can unofficially play old GB on any GBA-compatible using Goomba Color. Micro is awesome at what it does, I just hate that it has the Game Boy name yet can't play GB games and had yet another different link cable and charging port because Nintendo!