I used the Turtlewax one on my 20 year old headlights. It worked fine. It comes with three grades of sanding pads, polishing liquid and sealing wipe.
The kit says to start with the polishing liquid and if the result isn't satisfactory, go through with the sanding pads. Basically you take out all the yellowing and flaws with the coarsest and work your way up. Just like body work.