Stacy asks via Twitter:
@thefoodgeek What's the difference between ginger beer/ginger ale, and why don't I get carded when I order?— Stacey Jaros (@srj68) January 26, 2012
When we talk about ginger ale and ginger beer, we are essentially talking about two different things. Ginger beer is a brewed beverage that is made with essentially the same process as beer; ginger ale is a soft drink made with carbonated water and syrup. Ginger ale doesn't get you carded for the same reason RC Cola doesn't get you carded. Ginger beer is somewhat more interesting.*
Back in the day, ginger beer was a common drink to make at home. You would get, often from your neighbors, what was called a Ginger Beer Plant. This "plant" wasn't a plant, per se, but more like a sourdough starter culture for breadmaking. There are variations in structure from household to household and region to region, but the key elements of this plant are a bacteria and a fungus that have learned to live in happy symbiosis. For those intensely curious, the fungus is Saccharomyces pyriformis and the bacteria is called Brevibacterium vermiforme. Together they make something that looks kind of like squishy, wet rice. You would add the Ginger Beer Plant to water, sugar, lemon juice, and ginger and it would ferment and make tasty ginger beer.
Nowadays, it's less likely that you're going to find Ginger Beer Plant. There are places that have the "plant", and if you're interested in brewing ginger beer, there's no reason not to order some off of the internet. That being said, brewer's yeast is a popular substitution, because it's a lot easier to find, and you can even use bread yeast.
The other reason that you don't get carded with ginger beer is that it's been regulated in Britain since 1855 to be no more than 2% alcohol. The limitation of the amount of alcohol is accomplished by the lemon juice that I mentioned above; the acidity of the lemon juice slows the fermentation process, and ultimately caps how much conversion of sugar to alcohol and carbon dioxide can occur. Because of the low alcohol content, kids got to enjoy the drink.
*-Although modern, canned ginger beer is pretty much carbonated water with syrup as well, but there's at least one traditionally brewed still being manufactured.