I tend to agree with the natural relicing idea, but the reality is that modern pickguards wouldn't look aged until about 25 years!
One time, I had accumulated a bunch of white strat pickguards in a drawer and decided to 'age' them, then sell them. Here's the method that worked the best...
KMC is right about the steel wool. You need to gently take all the glossy shine off the pickguard for any dye to adhere to it. Do it till the plastic is dull white.
In a big wok or pot, heat up a mixture of tea, coffee, tobacco, and a few spoonfuls of yellow curry. You can buy small bags of curry at the grocery store for $2. That's what helps to give it the nicotine look. Use a lot more brown in the mix than curry yellow. Use just a spoonful or two of curry, or the guard will turn canary yellow.
DO NOT SUBMERGE THE PICKGUARD IN HOT WATER or it will warp. Wait till the water has cooled off, then let the pickguard soak in it for a day. Don't worry about getting the pickguard TOO colored.
The final step is to tame the dye by wiping the pickguard with pure bleach to bring the shade of tan/yellow down to the desired shade. I like to make the pickguard just a bit overdone, because over time the dye fades from the light. What you end up with is a pickguard that actually looks 40 years old.