Q. I know your column is for parents, but I'm having trouble with mine. My mother tries to run my life. She always wants to know if I've done my homework, where I am going, and now it's how far do I let a boy go on a date?

I don't think my sex life is any of her business. I'm old for my age (16 in September), which is old enough to decide when I can start having sex. From what the guys say at school, there's hardly a virgin in my sophomore class.

I've never had a serious boyfriend, but if I do fall in love I think I can do what I want with my own body.

I hope you agree, so I can show your letter to my mother.

A. You're right: Your body belongs to you alone, and so does your mind. But if you think you're old enough for sex, you have to be old enough to handle the consequences.

Some consequences may be minor; others can change your life.

It might embarrass you, for instance, to hear the guys at school include you when they name the girls who put out. This could hurt, especially if your boyfriend said he wouldn't tell.

Even if he were discreet, sex could escalate your investment in each other enough to break up the romance, if one or both of you isn't ready for a major commitment.

It could upset your relationship with your mother even more.

You feel alien to her now, and adventures with sex could put a new -- and a much higher -- barrier between you. I even could create a barrier within yourself, dividing you from the person you once were. For better or for worse, childhood would be over. For some, this is all right; for others, there is a terrible sense of loss.

Having sex before you're emotionally ready also could make you feel guilty, especially if you have been reared in a home -- and a church -- where sex is saved for marriage. You may say that's old fashioned and then find your conscience doesn't agree. This is particularly true if sex should become a routine part of a Saturday-night date. When you give your body away without your heart, you lose a bit of your integrity every time.

There is also the disappointment factor. Despite all the hype from Playboy and Penthouse, sex can be depressing to a teen-age girl, since many teen-age boys aren't very good lovers. Their girls may feel used or undersexed.

There is STD -- the label for a half-dozen sexually transmitted diseases, including VD. While it's unlikely that you would catch anything, each partner you have -- and each partner your boyfriend has had -- shortens the odds. There are an estimated 10 million new cases of STD each year.

Some (like gonorrhea) are a first hard to detect in most girls. One disease (Herpes Simplex II) is still impossible to cure.

But no risk is as great as pregnancy.

More than a million teen-agers get pregnant each year, even though most know about contraception. The experts say this may happen because they need more love than they get at home; because pot or alcohol may remove inhibitions, or because an aura of magic surrounds many young teenagers. Somehow there will be no accident on the highway -- no matter how drunk the driver -- and the purity of the marijuana can be trusted, even if the dealer cannot.

And certainly there will be no baby if you make love just this once . . . or maybe if you wish upon a star. Unfortunately, that's not ture.

Half of the first-time teen-age pregnancies occure within six months after a girl starts having intercourse. One out of five begins in the first month.

Even though you would use contraception, it wouldn't be long before there would be a "heavy, heavy hangs over your head" every month as you checked of the days. As you will discover from the mistakes of your friends, abstinence is still the only foolproof method of birth control The best precautions allow a 1-2 percent chance of pregnancy, while others have a failure rate as high as 10-20 percent.

If you did get pregnant, the consequences would be much more serious.

One option would be abortion. You might think it is just a simple, safe operation (and it is), but the moral issue is far harder to handle, even with counseling. When nature starts a body stirring, it's hard to be cool about it, especially when there are so many interpretations of "right-to-life." a

For some reason, people who are against abortion often favor capital punishment and bigger bombs, while those who are for it are usually against handguns and seal hunts. Only you could make the decision, but you might have to make it before you know where you stand, which could cause as many psychological repercussions as a full-term pregnancy, if not more.

Aside from this great moral choice, there are other considerations: the visit to the doctor or clinic -- always embarrassing -- the arrangements to be made and the $150 to be raised.

If you should decide to carry the child, you might marry, but at 15 or 16, the likelihood of success is pretty poor. So many teen-age marriages end in divorce.

If you didn't marry, you would have to decide it you could stay in school, how you would pay for pre-natal care and delivery and if you should (or could) live at home, or go to a place for unwed mothers.

You then could decide to keep the baby -- if you thought you had the financial and emotional stamina to care for a child alone -- or sign adoption papers: another wrenching choice.

Before you decide to have sex with your first serious boyfriend, please make sure it's not just another way to rebel. Even the right decision, made for the wrong reason, would be a mistake, and in this case, you stand to pay the highest price.

If you could talk with you mother openly, you probably would find that she asks all those questions because she cares deeply about you and has had to live with some consequences herself. Her own experience, as well as her heritage, has given her a set of values that she wants to pass on to you. They're important to hear. Values, after all, are the only real legacy a parent can give a child.

To consider them more deeply, there is a book called Growing Up With Sex by Richard Hettlinger (Continuum, $4.95). To learn about your own desires and feelings, read Changing Bodies, Changing Lives by Ruth Bell and others (Random House, $7.95). This is an extremely frank, accurate and complete book of whys, ifs and how-tos with many candid interviews with teen-agers. The graphic way they think and talk may shock your mom, but could help her understand you.

You also would get a clear message: Sex is not obligatory. A new study says it isn't even commonplace. Seventy-eight percent of the 15-year-old girls in the country have never had intercourse, no matter what the boys in your class say. And another 12 percent have only tried it once.

But if you do decide to be sexually active, remember that it is an adult's decision, so you must make the arrangements yourself, without your mother's help. Although your boyfriend would be equally responsible for a pregnancy, you're the one who would conceive, so you must see a doctor to protect yourself. To get advice on birth conrol, call 942-6006, the day-and-night answer line sponsored by the Center for Population Options. And please: Always go to a doctor or clinic if you get strange aches and odd sores, discharges or rashes.

And now a question for you: Are you really going to show this column to your mother?