Dr. Ruth Westheimer would like to have a word with the growing number of millennials who say they're too busy, stressed or cash-strapped to go on dates.

"Don't be stupid. Make sure that you have time for sex," says the no-nonsense sex therapist, now the subject of the new documentary "Ask Dr. Ruth." "Here is an activity that is so enjoyable and it's free. Make sure you have a relationship, and don't fall into the category of people who have lost the art of conversation.

"I'm all for dating apps, as long as they use them intelligently," she adds. "People on apps can lie and say they're 6 feet tall, when they're only 4-foot-7 like me."

These are the types of priceless, practical kernels of wisdom that Westheimer, 90, has been sharing for four decades as America's most celebrated sex guru. She rose to fame in 1980 with her call-in radio show "Sexually Speaking" and quickly became a spunky fixture of daytime and late-night TV, where she doled out her candid advice.

"Ask Dr. Ruth" (now playing in select theaters nationwide, streaming June 1 on Hulu) explores how Westheimer's tragic past shaped her into the tenacious woman she is today. Born in Germany to a Jewish family, she was sent to a Swiss orphanage at age 10 as part of the Kindertransport rescue effort, after her father was taken by the Nazis in 1939. She stopped receiving her parents' letters two years later, when it is believed both were killed in concentration camps during the Holocaust.

At 17, she moved to Palestine and trained to be a sniper for the Israeli Army but was seriously injured in a bomb blast. From there, she moved to France and then to the U.S. to study and teach psychology, eventually narrowing her focus to sex education.

"I would have never predicted that I'd be a sex therapist," says Westheimer, who credits a part-time job at Planned Parenthood in the 1960s with kindling her interest. "At first I thought: 'What is wrong with these people? All they do is talk about sex – not about the weather or politics or literature.' But maybe 48 hours later, I said: 'Hold it. What an interesting subject matter!' "

Westheimer is a trailblazer in her field, long emphasizing women's pleasure and consent, and fighting stigmas against homosexual sex during the AIDS crisis of the 1980s. Nicknamed "Grandma Freud," she has also become an unlikely pop-culture icon, appearing in movies, sitcoms, commercials and even her own board game.

But she has always been notoriously private about her personal life, including her three marriages. (Her third husband, Manfred, died in 1997 after more than 30 years together.) She was initially reluctant when filmmaker Ryan White (Netflix's "The Keepers") approached her about making a documentary, given her desire to keep work and family separate.

Westheimer eventually warmed to the project as a way to share her refugee experience with others, but also to "tell Holocaust deniers to stop denying the Holocaust," she says. "There are people who say: 'Look, that happened so long ago. Stop talking about it.' So I have to combat that."

Westheimer, who turns 91 next month, is still as sprightly and as busy as ever. She published three books just last year and attended the movie's premiere at Sundance Film Festival in January, where it received overwhelmingly positive reviews from critics (92% "fresh" on Rotten Tomatoes), drawing comparisons to last year's Oscar-nominated documentary "RBG."

"I'm really fortunate that I'm healthy and I love what I'm doing," she says. "I tell older people not to retire, but to rewire: They don't have to stay in the same profession that they are and can do something else. But don't sit home and be bored."

She's coy about whether she's dating now ("Next question," she says, laughing). But she speaks fondly of her two grown children and four grandkids, whom she spends any free time with at home in New York.

"I'm the proudest – write that in big letters – of my terrific four grandchildren, even when they beat me in chess," Westheimer says. "Mostly I take them to dinners or a show. I'm not a grandma who bakes cookies."