Financial columnist Sylvia Porter receives stacks of correspondence each day at her Fifth Avenue office here. Analyzing the contents of this outpouring over the past few years, Miss Porter and her assistants noticed a constant stream of inquiries from teen-agers and young adults. They were asking basic ''survival economics'' questions. What was withholding tax all about? When should one open a bank account? How could one obtain a credit card without first having credit?
To Miss Porter, who has been writing a syndicated financial column since 1947 , these questions indicated both a shortcoming in our educational system and the escalating complexity of money matters. Financial institutions are being deregulated and a plethora of services are being offered by banks, brokerage houses, and department stores.
''I think it is imperative that economics education be required in your senior year of high school,'' Miss Porter said in an interview, ''because by then you are old enough to have a sense of value - and of what is valuable.'' While she is encouraged that economics education is, in fact, increasingly being offered in school districts throughout the nation, she sees the growing complexity of money matters in our lives as reason for a new self-help book for young adults: ''Sylvia Porter's Your Own Money'' (Avon Books, $12.95). In addition to the book, in October Miss Porter will launch a bimonthly financial magazine called ''Sylvia Porter's Personal Finance Magazine.''
''The book focuses on young adults, but it's really for all adults,'' Miss Porter says. ''What I tried to do here was write it so simply that I would fall asleep under the simplicity of it. Actually, I don't expect teen-agers to be the major buyers. I expect their parents to be - as Christmas or graduation gifts.''
Though the author's attempt to speak in the vernacular of the young is sometimes strained, the book nevertheless is a simple, comprehensive guide to money matters. Because it is structured as a reference book, its 800 pages need not be seen as daunting.
Miss Porter begins by suggesting that young adults who live at home renegotiate their allowances with their parents. She suggests that parents who don't provide an allowance consider instituting one, since this is a valuable tool for young people to begin to learn financial responsibility.
''If you don't get an allowance, you probably have to ask for frequent handouts - which becomes tedious and irritating for all of you,'' the opening chapter of her book says. ''An allowance helps you manage your own money and gain experience in handling larger sums as you grow older. You learn to plan your expenses and to make your own choices about what to buy and what to save for.''
Besides basic guidelines on banking, saving, and credit, the book includes sections on shopping, fashion, health, personal safety, and many other topics not directly tied to money matters. Some of these items may seem out of place in this book, but Miss Porter's longtime argument has been that there is a financial component to virtually everything a person does.
''Do you realize how important it is for a girl or boy to know how to shop, where to go?'' Miss Porter asks. ''Look what young women spend on cosmetics. You spend a fortune on looking good. You need to know what a discount store is. There are certain things you can buy to get the designer tag, if you must. But should a 17-year-old going out on a special occasion buy the best dress or shoes in a store to wear once and never again? What I say is you go and buy something pretty, attractive, and cheap.''
The book guides readers through the complex aisles of the ''financial supermarket,'' explaining stocks (avoid speculation; invest in companies with good ideas and able management), municipal bonds (a good, long-term way to protect money, but perhaps too conservative for young people who are seeking to increase their capital), collectibles (avoid so-called ''limited edition'' items and stay away from precious metals unless you intend to devote much time and attention to the price of gold and silver).
Though Miss Porter encourages young people to be prepared for the advent of one-stop financial shopping, she thinks this may neither be a good idea nor a long-term trend: ''Just because Sears has an outlet for stocks, an outlet for insurance, for real estate doesn't mean they have experts in those fields. They may, but they may also just have little desks that say 'broker' or 'insurance salesman.' What matters is who sits behind the desk and what he or she knows. If you can help me, that's where I'll go.''