STEPHANIE SY: While cancer deaths in the US have decreased in recent years, experts say one age group has seen an overall rise in cancer rates younger Americans.

William Brangham has the latest on the shifting demographics and what can be done to address it.

WILLIAM BRANGHAM: They're called early onset cancers, which means cancer among adults under 50, and they are on the rise.

A groundbreaking report from the American Cancer Society looked at rates of 34 different kinds of cancer over several decades and found that 17 of them were more prevalent in millennials and Gen Xers.

So what is going on here?

Doctor Karen Knudsen is the CEO of the American Cancer Society.

Doctor Knudsen, thank you so much for being here.

I think the findings in this caught a lot of people by surprise.

When you look at the overall study, what stands out most to you?

DR. KAREN KNUDSEN, CEO, American Cancer Society: Well, you know, there are some surprises here, but in fact, we've been seeing some early indicators about this rise in cancers at an earlier age over the last several years, early onset colorectal cancer, I think, was the canary in the coal mine here, where we saw declining incidents in populations in the 65 and above, but rising in those that are 50 and younger.

These are ages for which we previously not thought about someone being at risk for colorectal cancer.

So clearly, something is changing, and this new study highlights that with 50 percent as you said, of the studies that of the cancers that we looked at on the rise in Gen xers and Millennials as compared to Baby Boomers.

WILLIAM BRANGHAM: Do we know why this is happening?

Because, again, the traditional I'm no oncologist, but the traditional understanding is older people tend to get more cancers.

You get cancer as you get older.

Why is this happening with younger people?

KAREN KNUDSEN: So what that is true, not for all, but for most cancers, age is a risk factor.

So that is without question, true.

But when we look at these cancer rates, they are sometimes two to three times higher in incidence for the Gen X and Millennial population as compared to Baby Boomers.

So we look at those data, we think that the typical risks are still at play.

Obesity.

10 of those 17 cancers are linked to obesity.

Lifestyle.

So sedentary lifestyle, of course, increases cancer risk alongside an unhealthy diet, lack of fruits, vegetables, grain fiber, et cetera.

So we know that those typical cancer risks are at play.

But there must be something else, because these rates are so different as compared to the previous generation, the baby boomers.

WILLIAM BRANGHAM: One of the things we also are concerned about is, if you get cancer, whether or not it ends up ending your life.

What do we know about cancer death rates?

KAREN KNUDSEN: That's right.

So when we actually looked in these three different age populations, we could follow cancer mortality as well in a subset of them, and we saw five actually that were of increased mortality rates in Gen X and Millennial populations.

It was liver cancer, specifically for women, uterine cancer, gallbladder cancer, testicular cancer, which is not a cancer of aging, and colorectal cancer, which we talked about.

So these leave then open questions for which more cancer research will be needed in order to address and we can speculate as to what are some of these additional exposures that someone may have been subject two, or could it be a compendium of exposures, diet and lifestyle?

What I would say is that what we know right now is of all the cancers that we track every year, our own research estimates that up to 40 percent of them are preventable due to behavior modification, things like having that healthy diet, staying active, maintaining a healthy body weight.

Of course, don't smoke, limit alcohol and get screened.

So we know that early detection saves lives, and that increase, that increased survivorship is, of course, something that is well within means for screenable cancers.

WILLIAM BRANGHAM: On that issue of screening, given what this study reflects, do you think we ought to be changing the guidance that we give to not just to doctors, but to individuals, as far as when you get checked?

How often you get checked?

KAREN KNUDSEN: Such an important question.

So it's important to remember that screening is not just about your age, that's one portion, but it's also about your genetics, if you know it, your family history, your own medical history, and your risk of exposures.

So never too early to when you are at your physician, ask and take ownership of it on your own.

What is the right screening plan for me, so that all of those different criteria can be taken into account.

Now, as relates to cancer screening guidelines, we of course, look at this regularly.

We are in the process right now of rewriting our prostate cancer screening guideline, but we were the first at the American Cancer Society to drop first colonoscopy from age 50 for people of average risk to age 45 because of these trends of early onset.

So we will continue to monitor and determine whether or not the screening guidelines require additional modification the science will lead us.

WILLIAM BRANGHAM: Do you think younger people are getting the news about this?

Do you feel like that this message is percolating out into society?

KAREN KNUDSEN: I think that they are and hearing reports like this can help them take agency over their own health, those prevention behaviors.

We talked about how this discussion about screening, but also don't ignore symptoms.

So if you're 44 years old and you're experiencing something that seems like it could be associated with colorectal cancer risk.

Don't wait.

Be seen and have that discussion with your physician, even if you're too young to technically have had your first colonoscopy.

Don't ignore symptoms.

But it's also important to note that on balance, there's good news as well for cancers overall since 1991 which was the high water mark for cancer in this country, we have reduced the overall cancer mortality rate by 33 percent so there is more hope than ever before, which is altogether more reason to not ignore symptoms, take agency over your own health, for prevention, but also having symptoms addressed.

WILLIAM BRANGHAM: Like that there's at least a little bit of good news in that.

Doctor Karen Knudsen, CEO of the American Cancer Society, thank you so much for joining us.

KAREN KNUDSEN: My pleasure.

Thank you for having me.