I have this old bed that creaks, how can I stabilize it?

Is plywood support that you added screwed to frame? If not, it may help to screw plywood to frame. Pre-drill first. Also, you could add plywood to underside of frame. Cut some plywood "triangles" 6 or 8 inches, and screw to underside of frame at corners. Pre-drill always with old wood. There may be weakness where legs attach to frame. Much more difficult connection to repair. Lots of glues and fillers out there for that purpose.