In the middle of what she calls her third act of life, Jane Fonda plays Nancy Reagan in a upcoming film, and she appears in the television series “The Newsroom.” (Angela Weiss/GETTY IMAGES)

Jane Fonda has been in the public eye for more than 50 years, as an Oscar-winning actress for the films “Klute” and “Coming Home,” an activist against the war in Vietnam and an exercise guru who has made more than 20 workout videos (now DVDs). Her first video came out in 1982 and helped start the aerobics craze.

Now 75, Fonda is in the middle of what she calls her third act of life. She has a boyfriend, music producer Richard Perry. She’s still friendly with ex-husband Ted Turner. She had children with ex-husbands Tom Hayden and the late Roger Vadim. A few years ago, she said in a recent telephone interview with The Post, she realized she was happier than she has ever been. “It took me by surprise because I come from a long line of depressives,” she said. “I wasn’t very happy as a younger person, yet I found myself happy.”

Fonda is working as much as ever, too. She plays Nancy Reagan in an upcoming film about President Ronald Reagan’s butler, and she appears in the television series “The Newsroom.” Her newest book, “Prime Time,” a mix of advice on health, fitness, friendship, sex and other topics, recently came out in paperback. And Fonda just released a new yoga DVD. She says she hopes to write a few more books, one on adolescence.

You have a new workout DVD, “AM/PM Yoga for Beginners.” Tell me about it.

My last book, “Prime Time,” is about aging successfully. I did research for that for three years. While I was doing that, I was taken by surprise by the extent to which gerontologists feel that staying physically active is maybe the number one, most important [thing] for people to do. Who better than me to do it?

The exercise DVDs that are out there are not aimed at my demographics, baby boomers. I am older, and I have had a knee replacement and hip replacement. I have not made a secret about that, so people could understand [that] if I can do these things, then they could do them, too.

What is your exercise routine?

I live on a hill. I walk down the hill and I walk up. It is a very challenging walk. If I am near mountains, I like to hike. If I don’t have time to go out, I do a recumbent bike, the elliptical or treadmill. I alter them because I get bored after about 10 minutes. I do that while I watch television. Then I do weight work, either with dumbbells or with the bands. I do at least 30 minutes, and I try to do it five times a week, if not more.

As you get older, you lose muscle mass, so it is important to keep working your muscles. It is important because we want to stay independent. We want to be able to get in and out of cars, out of chairs and couches by ourselves. We have to maintain a strong body, strong back and strong thighs.

After I did this yoga video, I had a pretty intense back surgery. As I was recovering, I am thinking, “Thank God I have remained strong, because everything goes into the quads.” I couldn’t use my back or stomach. I am well now, but for four or five months everything depended on my having strong legs. That is what allowed me to be independent, and that is really important for older people.

The only other thing I do is dance. My boyfriend and I dance as much as we can.

What do you eat?

I am not a vegetarian, but I eat carefully. I eat a lot of fresh fruits and vegetables and fish and chicken. A couple of times a week, I will eat red meat. I try to eat color. Every day I try to have something dark green or something dark purple, like blueberries. I had a big bowl of blueberries today. I have broccoli for lunch. Along with protein. I try to eat as much fish as possible.

You talk about your life as lived in three acts. What has gotten better in the third act?

First of all, let me say that when I was in my mid-60s approaching 70, I realized I was so happy. It took me by surprise because I come from a long line of depressives. I wasn’t very happy as a younger person, yet I found myself happy. That’s not what I expected. It turns out through very extensive studies of hundreds of thousands of people that over-50s — men and women, married, doesn’t matter — have a sense well-being. They are less stressed. They are less hostile. They have less negative emotions. They tend to see what people have in common rather than the differences, which is why we become good mediators.

This is very important for people to know. We have to get the word out so young people won’t be so scared about getting older.

Obviously, if you are suffering in poverty or have a terrible illness, this trend is not the case. Most people — if you don’t have an illness, but even some who do — we don’t make mountains out of molehills. We make lemonade out of lemons. We’ve been there, we’ve done that, but most of all, we know what we can let go of, what we don’t need. We become lighter.

What has become harder?

Moving fast. Lifting heavy weights. Opening bottles. I suffer from osteoarthritis. When I work in the kitchen, I have different tools that don’t hurt my hands. I have special things that help me open bottles and jars. You have to learn to adjust.

I used a BlackBerry for years until my thumbs gave out. There was no way I was going to use an iPhone. I thought, “I can’t manage it.” Well, reality forced me to switch to an iPhone because I can use my index finger tapping. I don’t have to use my thumb.

Bette Davis said aging isn’t for sissies. That is right. But trying to pretend that it’s not happening, that your body is the same as it used to be — no. I say, “Okay, this is the way it is. Let me find a way to adjust so I won’t hurt myself, and let me write about it so that other people know what to do.”

How has technology impacted your life?

I am a blogger. I tweet. I use Facebook. I am very much a part of the social network world. I am very glad, because I learn a lot from it. I write on a laptop. I carry my laptop everywhere. I am very comfortable with it. Although I started at age 71 — the blog, that is — I have been using a computer since I was 58.

[Former husband] Ted Turner doesn’t have a cellphone or a computer. Hasn’t a clue how to use them. A lot of people don’t. You don’t have to. But I am a communicator. I like to learn. I want to communicate. I have lots of ideas. I have nonprofits. I have things that I want to say. Being a viral person helps you do that.

Do you prepare for roles differently now?

I have always been someone who puts in a lot of time preparing. I think I am a braver performer now. I take more risks as an actor. I think I am a better actor than I was, because I know myself better and because I am a happier person. I left the business for 15 years — I was very, very unhappy in the ’80s. I just said, “I can’t.” Some people can act if they are unhappy. I can’t. I said, “I am just going to quit.” Then 15 years later, I was a very different person. I was ready to go back, and I find joy in it.

I prepare pretty much as I always did. I love playing Nancy Reagan [in the soon-to-be-
released movie “The Butler”]. I think I kind of look like her. I am told she is very pleased at the fact that I am playing her. Even though we don’t necessarily agree. But you know something when you are getting older? All that mellows. I am still very strong in my beliefs, but Ted Turner helped with that. His philosophy is you catch more bees with honey than with vinegar. And I spent 20 years in Georgia. I am much more comfortable with people who don’t agree with me. I can see them as human beings instead of saying, ”Hm, we’re different.” But part of that is just plain age.

How is your memory? Do you still memorize lines easily?

Memorizing has always been easy for me. That really helps me in the “The Newsroom” because speeches are very long. I memorize easily, and that stays true. Now, remembering things like where did I put that or who is the person’s name, that can go right out of my mind. The trick for that is to breathe and let it go. The worst thing you can do is fret over it. Maybe in a minute or 10 minutes or 20 minutes it comes to you. If I can’t find something, I don’t fret. I know it is going to show up in a pocket of a coat or a purse.

People say Hollywood is harsh for older women.

It is. I understand why that is true: It’s a big screen up there that you watch. And it is very nice to look at beautiful faces with beautiful skin and bodies.

[But] it is a business, and there are more and more older people in the world. It is the fastest-growing demographic globally, and within that fast-growing demographic, women are the biggest part of that. There are more older women than anybody. I believe in time [that] Hollywood — and all sectors of society — will have to adjust to this new reality.

But it is tough. I am [now] 75. It’s not easy. I hope in some small way I can change that, just by being there.

What’s in your future?

I want to keep working as an actor. I am very, very happy in “The Newsroom.” I think it is a fantastic television series. It is written by Aaron Sorkin. Have you seen it? I love playing that character, Leona Lansing. I want to have my own television series. I want to make movies.

Looking back, is there anything you wish you had done differently?

I wish I had been more confident. I wish I had had more confidence in myself and been more conscious. I always used to think that being self-conscious was a pejorative. The person who taught me otherwise was Katherine Hepburn when we were making “On Golden Pond.” She was extremely self-conscious, and what that means is conscious of how you put yourself out to the world, conscious of the way you are received by people, the way you are perceived, both in how you look and how you are experienced by other people. She got mad at me because I wasn’t conscious enough. I didn’t pay much attention to how I looked.

I am in the process of putting together a reel of photographs of my life for my birthday party. I am in a room where the walls are covered with [photo] albums, and I am going through the albums. It really strikes me how little attention I paid early on to how I came across, how I looked, how I was in the world. It is striking. I think if I had been more self-conscious, I would have made fewer mistakes.

How do you project yourself now?

I have become a happier person. I have become a wiser person. I am grown up. It took me a long time. I am a late bloomer, that’s for sure. That is what I project, someone who has gained a certain amount of wisdom, who is good in her skin. I don’t get rattled very much, frankly.

Do you think about what you wear and how you look?

Yes. I am far more glamorous now than I was in my 20s, 30s and 40s. Way more. I don’t obsess about it, but if I have to be in public, I am going to go out of my way to look good and glamorous.

I think the most important thing is to be confident about yourself. Older women tend to be more confident. They know who they are. I think that can radiate from you. That will be destroyed if you overdo plastic surgery, if you try to look like someone who is way younger than you are. . . . I have had plastic surgery. I don’t want to be a hypocrite. I have said to the surgeon, “I don’t want to get rid of the wrinkles. Take the bags away from my eyes.” I still look like me, whereas I know too many people I see walking toward me [where] I know somewhere in that encasement is someone I once knew, but I don’t know who they are. That’s not good.

Staying healthy, staying fit, being confident, being intentional about how you live, staying curious about things, maintaining love — it can be sexual love or it can be the love of friends — this is very important as you get older, and I think it helps people glow from the inside out.