Also known as simply "SDA," SD Alcohol is specially denatured ethyl alcohol, which means it has been rendered undrinkable and is thus not taxed in the same way that consumable, distilled spirits are. Usually, "SD Alcohol" is listed on the ingredients as "SD Alcohol-XX," where XX is a number indicating what denaturing formula was used. There are many ways of denaturing alcohol, most of which consist of adding undesirable (usually repulsive to the tongue), highly toxic chemicals to the mix. Common denaturants include phenol (the same stuff they spray your biohazard suit with after having been in a high level infectious disease facility, which is also the same stuff you spray on your throat in dilute form as ChlorasepticTM in order to kill your throat's nerve endings), methyl n-butyl ketone, mercuric iodide, and brucine sulfate. In some cases, essential oils extracted from plants can be used as a denaturing agent, although this doesn't make it safe to drink.
The reason for this is obvious; ethanol has various uses, many of which aren't recreational...so it would be unfair to "sin tax" it in all cases. It must be denatured before it leaves the distillery, or it will be taxed like any other liquor. SD Alcohol is found in many things that you put in your mouth, including mouthwash and "Sweet BreathTM" breath freshener. This is why it is very unwise to drink your mouthwash.
If you're still not of age, and are desperate to get drunk, I highly recommend that you avoid anything that contains SD Alcohol. You will be drinking ethanol, but you'll also be introducing various nasties into your body that are far more toxic than even the ethanol itself. The point of denaturing is to make ethanol completely repugnant to human beings.
Just remember this: any alcohol that is actually intended for ingestion (not meaning just "in your mouth" but "in your stomach") will not be denatured. This means that extracts intended for cooking are relatively (ethanol, after all, is one of the most toxic recreational drugs in existence) safe to drink. Vanilla extract usually contains 70 proof (35% alcohol), but the real kicker is lemon extract. Although it's pretty disgusting, you can easily convert a two ounce bottle of lemon extract (which is usually 160-170 proof) into two 12 ounce quantities of somewhat gross 13 proof "hard lemonade." I'll admit that I tried it, and I only had maybe a week to go before my 21st birthday. I just wanted to see what it was like, damnit! I'd recommend waiting until you can purchase the real deal.
Note that there is a huge difference between "specially" denatured alcohol and simple "denatured alcohol." Denatured alcohol, or more specifically, "completely denatured alcohol," is unfit for use in or on the human body whatsoever. Specially denatured alcohol can be used in the mouth and on the skin, as long as it isn't ingested via the stomach. There are various grades of industrial denatured alcohol, including "reagent grade" and "solvent grade." None of these can be used in or on a human body, and are instead used as solvents or additives to gasoline (sometimes they are even denatured with gasoline).
Another note: I received a message regarding my Chloraseptic factoid...As far as I know, the active ingredient in plain Chloraseptic has always been phenol, which is also known as carbolic acid, or phenyl alcohol, or phenic acid. However, there IS a new formula used for "Ultra Chloraseptic" that contains the local anesthetic benzocaine as an anesthetic, and plain, non-denatured ethanol as an antiseptic.