When I first saw the title "Sign Language: Not Just for Kids" the first thing that popped into my head was, "You've got to be joking. Who thought it was just for kids in the first place?" Blame my upbringing, I suppose...as the homeschooled child of a sign language interpreter, I learned the beauty and widespread use of sign language at a very young age.
First off, to say that sign language is simple is to say that the Russian or Japanese spoken languages are simple - to someone learning the language, it simply isn't true. There are many different sign languages worldwide just as there are different spoken languages and each is just as detailed and complex as any spoken language with all the expression and inflection you'd put into your everyday speech. As with any other language, the sign languages can tell you so much about the culture and thought processes of their "speakers" and help broaden your own worldview as you learn.
In the US, the most widely used non-spoken language is ASL (American Sign Language). Many people who take sign language classes from community colleges and high schools do not learn ASL, though there are such classes available in most areas, but rather learn a sign system such as SEE2, or Signing Exact English. The first thing to realize is that ASL is not English and does not adhere to English rules, whereas the SEE or SEE2 systems take English words, thought and word order and translate them into motions. To fully understand sign language, you must know which sign is a separate language and which are simply turning a spoken language into an unspoken one.
Through my life I've had the privilege of knowing many people who were deaf, Deaf, hard-of-hearing or CODAs (Children of Deaf Adults). With the exception of the last one (though some CODAs also fall into this category), all of these people are classified by the government and society as "hearing impaired". True, there are some deaf people who consider themselves impaired, but many see the term as an insult to their culture and way of life and do not see deafness as any kind of impairment. Essentially, this is the difference between deaf and Deaf; the first simply can't hear and may feel handicapped by it, while the second identifies with the unique culture and language of the Deaf community.
I personally love languages and devote my time to learning as many languages as a I can as well as I can, yet those who have known me my entire life still struggle to remember that I am truly bilingual as I learned English and ASL side-by-side and had many classes in school that did not involve speaking out loud. I read some statistics that classify ASL as the third most commonly used language in the United States behind English and Spanish, so to me it makes perfect sense for anyone wanting to learn a language to consider ASL and to really put some effort into finding a qualified teacher and/or native speakers to learn from. Yes, the language developed so that people who are not hearing or people who cannot speak can effectively communicate with each other, but just because you may not be one of these doesn't mean you won't benefit from learning the language as well.
As I sit here thinking of all those cutesy Baby Sign books and teachers who show elementary school children how to sign the alphabet I suppose I can see where this notion of sign language being for kids came from. Most of the time children are taught basic vocabulary and, often, how to form sentences in a sign system such as SEE2. Just because I can speak a few words of French, doesn't mean I can speak French and just because children are taught basic vocabulary doesn't mean they know anything of sign language. Do I support children learning these basics? Absolutely. Do I think it should stop at that? Absolutely not; children who wish to learn sign language should go into it with the understanding that what they have already learned is not sign language and that learning the actual language will be an exciting and beneficial experience.