We're a nation living longer and longer.

Over the next 30 years, the number of Americans age 90 and above is expected to triple.

And an NIH-funded research study, called 90PLUS at the University of California, Irvine, is trying to learn all it can, right now, from a group of men and women who've already managed to get there.

Six years ago, we first reported on their first set of findings, factors associated with longer life, exercise, moderate drinking of alcohol and caffeine, social engagement, and our favorite, putting on a few pounds as we age.

The 90PLUS study's focus is now on memory and dementia.

What they've learned, and what they haven't, drew us back, as did the 90PLUSers.

Take a quick look at when we first met them in 2014.

My birthday is February 7th, 1918.

I was born on August 25th, 1920, and I'm 93 plus.

June 15th, 1918, and it was, I'm sure, a lovely day.

The men and women we met six years ago had all agreed to be checked out by the 90PLUS study team, top to bottom, every six months.

Big smile.

Their facial muscles.

Excellent.

How they walk.

How fast they can stand up and sit down.

Fantastic.

And critically, how they think.

Now spell world backwards.

D-L-R-O-W.

Three?

They were an impressive and active group.

A B-17 gunner in World War II.

A fellow World War II vet who drove a convertible.

A 95-year-old speed walker.

Ballroom dancers.

I asked them, aren't you going to ask us any questions about our sex life?

And they said no.

And sadly, some who had begun to struggle with dementia.

What is today's date?

Today's date?

Mm-hmm.

Today's date.

What's the oldest person you have seen?

I've seen several 116-year-olds.

Neurologist Claudia Kawas, the 90PLUS study's lead investigator, says studying the oldest old is increasingly important.

Half of all children born today in the United States and Europe is going to reach their 103rd or 4th birthday.

Half?

Yes.

Half the children born today are going to live to 100?

To 103 or 4.

You know, I don't feel a day older than I was yesterday.

They invited us back six years later, and we found some study participants, like Helen

Weil, the ballroom dancer, thriving.

And I do like so 10 times.

Now 99, Helen showed us how she exercises in her chair.

Stuff like that.

How you doing, Jeff?

Good to see you.

What's going on, Lou?

Lou Tirado, the World War II gunner, turned 100 in August.

Lou was using Zoom.

When he was a kid, most homes didn't have a radio.

Do you have an iPhone?

I have an iPhone, yep.

You on Facebook?

Yes.

Do you use Siri?

Yeah, I tell her every evening, wake me up at 6.30 tomorrow morning.

And she does.

Yes.

Who is our current president?

President is Trump.

Who was the president before Trump?

Obama.

Because of COVID-19, the 90-plus study is doing cognitive tests by phone.

Subtract 7 from 100.

Lou and Helen ace them.

And keep subtracting 7.

93, 86, 79.

Her memory is better than mine.

But one of our favorite 90-plussers from six years ago, Ruthie Stahl, is not so lucky.

Back then, at 95, she was zipping around in her lime green bug.

I am flying all over the place.

But today, at 102, she didn't remember our having met.

What is your first name?

Leslie.

That's a nice name.

Thank you.

Ruthie is as charming and upbeat as ever, but her memory is failing.

The current president or the president before him?

I'll take either.

No, I can't.

Do you remember your parents?

No.

No.

Oh, my.

It's funny.

I don't remember them.

Is it frustrating when you can't remember?

No.

No.

It just passes on to something else.

Dr. Kawas says most people, probably even most doctors, would assume Ruthie's memory problems stem from Alzheimer's disease.

But scientists are finding out more and more about the complexities of what causes dementia.

You hear people say, she got Alzheimer's, he has Alzheimer's, when they really should say dementia.

That's exactly right.

Dementia is a loss of thinking abilities that affects your memory, your language.

It's a syndrome.

It's a syndrome kind of like headache is a syndrome.

You can have a headache because you've got a brain tumor, or you can have one because you drank too much.

And it's the same with dementia.

We were sad to learn that some of the 90-plus participants we met in 2014 have passed away.

But by donating their brains, as Ted Rosenbaum did, they are very much still part of the study, contributing some of its most fascinating and confounding results.

After a participant dies, the 90-plus team gathers to review mounds of data.

Now, because of COVID, they gather on Zoom.

Videos from visit two.

So tell me what you're going to do when you go home today.

Ted's test results showed years of memory problems, as we had seen six years ago.

Give me a hint.

The 90-plus team concluded that Ted probably had Alzheimer's disease.

But then awaited results from their collaborators, a team of pathologists at Stanford University who independently examined Ted's brain.

They don't know anything except the brain they've got in front of them.

And then you come together.

And then we come together, and it's like a reveal party.

The definition of Alzheimer's disease is having the proteins amyloid and tau, often called plaques and tangles, in the brain.

OK, the home stretch.

But when the Stanford team made their report, Ted's brain didn't have either.

As you may see, without even zooming in, the section is clear, it's clean.

We're negative for beta amyloid here.

It actually looks awfully good.

It actually does, yes.

You sit around, you look at that.

What do you conclude?

The only pathology we found in his head, actually, was TDP-43.

The story will continue after this.

TDP-43, a breakthrough.

It's a newly identified cause of dementia, a protein originally found in ALS patients that Kwas now believes accounts for up to one in five cases of dementia in people over 90.

Can you find out if you have TDP-43 while you're alive?

Not yet.

And you can't find out if you have two other dementia-causing conditions either, tiny strokes called microinfarcts that damage brain tissue and hippocampal sclerosis, a shrinking and scarring of part of the brain.

So it's likely that many people in their 90s who are diagnosed with Alzheimer's may actually have something else.

There's a whole lot of stuff that goes on in the brain that we have no way of diagnosing during life.

So we get a lot of those surprises.

But we also get surprises where people have an awful lot of pathology in their brain, a lot of Alzheimer's disease, a lot of TDP disease, and they still turn out to be normal.

Let me hold the chair for you.

That's what happened with Henry Tornel, Helen Wild's ballroom dancing partner, who joked about studying sex over 90.

Henry died at 100 of cancer, mentally sharp as ever.

We should all be so lucky.

But his brain told a different story.

Beta amyloid.

I don't even have to zoom in.

Fluoride, very positive, positive as well.

The Stanford team found the highest level of plaques and tangles and TDP-43, especially stunning, since more than one pathology typically means more severe dementia.

So he was a huge surprise.

He was one of our surprising 90-year-olds who managed to have good cognition in the face of things in their brain that should cause dementia.

It used to be that when a person like Henry, with clear thinking, was found to have plaques and tangles, scientists assumed dementia was just a matter of time.

But now they're thinking about it in a new way, that maybe certain people have protection against dementia, a phenomenon they're calling resilience.

To prove it, though, they need to follow people who are still alive.

Enter convertible-driving Sid Shiro from our story in 2014.

Sid had a PET scan back then for the study, which revealed significant amounts of amyloid in his brain.

The question was, would dementia be around the corner, or might Sid somehow be resilient?

Happy birthday to you!

Thank you!

Sid turned 99 this summer.

How old do you feel?

I always say 69.

Sid has circulation problems that affect his breathing, but his memory?

Well, he told us about buying his first car 80 years ago for $18 in a pool hall.

A 31 Chevy convertible with a rumble seat.

A rumble seat!

And I didn't know how to drive.

You won it in a pool hall.

Did you win it on a bet?

I didn't win it.

I bought it.

You bought it?

I gave him $18.

Who sold a car for $18?

He needed the money to shoot pool.

So I know he's got at least two pathologies in his head.

I know he's got, you know, probably high amounts of Alzheimer's, and I know he's got some vascular disease.

And we tested him just a couple weeks ago, and...

Good morning.

He did great.

Please tell me how many nickels in a dollar?

20.

How many quarters in $6.75?

27.

Wow, you are quick.

So, is that resilience?

I think that is definitely resilience.

It might be what resilience is all about.

Could it be a gene?

It absolutely could be, or maybe even more likely, multiple genes or combinations of genes.

Here's my observation.

Okay.

You knew more six years ago than you do now.

There are just so many questions that we don't know the answers to.

More questions.

That is really a brilliant observation.

And what science is all about.

For every new answer, two new questions.

For every new discovery, like TDP-43 dementia, and especially resilience, new mysteries to solve.

So, like its participants, the 90-plus study is keeping at it, trying to help the rest of us make it to age 102 with Ruthie's spirit, but memory intact.

It's a shame.

It's a shame.

Because there's a lot I could remember.

And I'll bet you had a wonderful life.

Oh, I have.

It's still going on.

Thank goodness.

Thank you.