Sex therapist Ruth Westheimer, better known as Dr. Ruth, rarely turns down an opportunity to speak about sex. In fact, she recently celebrated her 85th birthday by giving a talk, “The Art of Arousal,” at the Museum of Fine Arts in Houston. And she continues to dispense frank advice as she has since her first radio show, “Sexually Speaking,” in 1980.

The only difference today is that Dr. Ruth does it on her YouTube channel, Web site and Twitter feed. She has written 39 books, including “Sex for Dummies” and is at work on a new one. And this month, a play about her life, “Becoming Dr. Ruth,” began a run in Hartford, Conn.

“That’s pretty neat,” she said.

As a young woman, Dr. Ruth was a kindergarten teacher and pursued a doctorate in education at Columbia University. But a stint at Planned Parenthood piqued her interest in human sexuality. This spring, the organization awarded Dr. Ruth the Margaret Sanger Award, “for her lifelong commitment to empowering women and men to talk openly and honestly about sex and sexual health.”

The essence of her advice, she said, is the importance of relationships and family. Born in Germany, the only child of an Orthodox Jewish couple, she survived the Holocaust after being sent on her own to Switzerland in 1939 when she was 10 years old. She never saw her family again and assumes her parents died in a concentration camp. “Everything else, sexual enrichment, other positions, all can be taught. But first there has to be a relationship,” she said.

Rising to the occasion, Ruth Westheimer shares a hug with President Obama at Planned Parenthood’s national conference in Washington on April 26. (Pete Souza/THE WHITE HOUSE)

Dr. Ruth has been married three times (two divorces, one death) and has two children. She spoke with The Post in a recent telephone interview from her apartment in New York, where she has lived for half a century. Her trademark German accent, wit and frankness, are still intact.

How have you stayed so active?

I love being Dr. Ruth, so I have never thought of retiring. I also took to heart [this saying]: “Not to retire, but to rewire.” That means “Don’t retire and sit in a rocking chair, but do something else that is of interest to you.”

So in my case for example, something brand new: a book coming out this November. It is called “The Myths of Love” — m-y-t-h-s.

It’s the Greek and Roman myths, of love with my interpretation, like Leda and the Swan. Now, I don’t want to talk more about the book because it’s not out yet. However, this is the answer to your question. I find something of interest to do and I am fortunate when I make a phone call, people know who I am.

So life has been good for you.

I am fortunate. I am fortunate I am living in New York. I am fortunate I can afford car services to Washington Heights, where I have lived for 50 years in the same apartment, overlooking the Hudson. My daughter and family live 10 minutes from my house, and my son in Ottawa, one hour by plane.

My apartment was just decorated by Nate Berkus, the decorator: B-e-r-k-u-s. He used to work with Oprah. He decorated it when he had his television show, then I was on his show, before and after and on page 141 of his coffee-table book. It is called “The Things That Matter.” It shows how I collect turtles. Turtles, but not live turtles, but all kind of figurines.

Why turtles?

A turtle, if it stays in one place, is safe because it carries its house on its back. Nothing can happen to that turtle.

There is another reason for the turtle. “Turtle” in German is called “scischildkroete.” S-c-i-s-c-h-i-l-d-k-r-o-e-t-e. That is the brand name of the dolls that I used to have in Frankfurt before I had to leave my grandmother — my mother and father were already in a labor camp — so that symbol of a turtle connects me with my past and with my early childhood, which was a happy one.

So the answer to your question is: I don’t have to look for occupations, like bridge playing. I have nothing against people who play bridge, but I have enough interest in the things I do, writing books, giving advice, having a brand-new television show. Every week for 10 minutes on Shalom TV — it is a small cable station — they are showing it for commentary, “Wisdom of Dr. Ruth.”

Not everybody has the energy to be out every night. . . . I don’t let people call me before 9 a.m. I sleep from midnight until 9 or 8.30. These days I say, ‘Don’t call me before 10.’ [She laughs.] This way I have enough energy to be out there.

Do you exercise regularly?

No, but I did see something brand-new I like. I take a lot of taxis. In New York, the taxis now have a woman [on a video screen] who teaches exercises while you ride. She tells you to pick up your pocketbook with one arm, then the other one and count to 10. Sit straight. Pull in your stomach and count to 10. I love it. That means you are using your time productively. I sometimes use my time productively by sleeping. I tell the taxi since I live all the way uptown if I fall asleep, make sure you wake me. This is an even better idea.

Let’s talk about your advice. What is the biggest concern you hear from people about sex as they age?

The biggest concern — and correctly so — is having an erection for men. That is one of the reasons I came out with the wine [Dr. Ruth’s Vin D’Amour, a line of low-alcohol wines from California]. It has only 6 percent alcohol. Drink a little because everybody has stress; don’t drink too much. She falls asleep and he can’t have an erection. That is particularly so for older people.

I tell older people, “Don’t expect the same intensity of your sexual response. Engage in sex in the morning, when the testosterone level is highest and after a good night sleep for her. Have a little breakfast, hang the phone off the hook, go back into bed.”

There are some things that are common sense, but people don’t know them. That is why I did the book “Sex After 50.” We shouldn’t assume people know all that, even though we talk a lot about sex. There are things women need to know. My book “Sexually Speaking” has what every woman needs to know. I did it with a gynecologist, Amos Grunebaum. I did it purposely with a gynecologist because there are many medical questions. [As people age,] the orgasmic response is less intense. The ejaculatory response for men is less intense. I say, “Make the best of what you have.” Don’t tell me it used to be like this. I know all that. Make the best of what you can do now.

What about for your own self as a widow?

No personal questions! But I will tell you one funny thing. I was just in Washington. I got the Margaret Sanger award. I was kissed by President Obama. I haven’t washed my face since. So I tell the story about my late husband, Fred. He loved Diane Sawyer. One time, “60 Minutes” came to my apartment. I didn’t have the heart to say to Fred, “You can’t be home.” So we are sitting down and Diane Sawyer, the first question, she said, “Mr. Westheimer, how is your sex life?” To which he answered — and I have it on tape — “The shoemaker’s children don’t have shoes.”

So I tell everybody, in the Talmud, the Jewish tradition, it says a lesson taught with humor is a lesson retained.

Has your audience changed as you’ve gotten older?

The questions have not changed. There is a lot of loneliness. There is a lot of people divorced or separated or who have never found a partner.

And what people said is the same: No time, being tired, not finding the right person. The family doesn’t agree with that person. Some affairs. He found someone in bed with his best friend. And, and, and. The basic issues have not changed.

Is romance dead?

I think we have to talk more. I don’t think it is no romance, but not enough. I’ll tell you what else worries me. I see young people holding hands and with the other hand they’re texting. People are going to lose, and not just young people, older ones are going to lose the ability to converse, the ability of conversation. They are being fed the news all the time. They lose the ability to concentrate.

What’s wrong with that?

It will have an effect on the relationship. I do believe a good relationship — I am not against people having a phone; I have an iPhone — but I want people to not lose the ability of interpersonal relationships. When I want to be with someone, I want to touch them. I don’t want them to be on the Internet.

It doesn’t mean I don’t want older people to converse and see their grandchildren. What I would like is a balance.

What about the increased rate of HIV/AIDS among older people?

Yes, they think — and young people, too, they think — they don’t have to worry. They don’t see AIDS patients walking around. They don’t have friends who have died. And if they get AIDS, there is a medication. I see a rise in indiscriminate activity among young people and among older people.

What do you think about that?

I think we have to talk [about how] not to be stupid and to be careful.

First, get to know each other before they go to bed. Get to know each other, even for older people. If you are aroused, go home and masturbate until you know the other person is safe. You can use your hand or use a vibrator. I even endorsed an eroscillator — e-r-o-s, like “eros” — because the man who came up with it is a scientist from Switzerland who came up with the electric toothbrush. I thought if he came up with the electric toothbrush, I can endorse his eroscillator.

What do you hope your legacy will be?

I think people will say she had the guts — in Jewish tradition, it’s called chutzpah. She had the nerve to talk about things other people were too worried to talk about. And she hopefully helped prevent unintended pregnancies and she helped some people to have a better sex life, until the late stage in life. Even have orgasms.

I don’t mind if people say they get aroused by my radio program; I think that is great, I provided you with foreplay. But take it seriously. Don’t let boredom creep into your bedroom.