When do we get old? People age 18 to 29 say "old age" starts at about 60. But those in middle-age figure it starts at 70. And those 65 and older put the threshold at 74.

So it goes with other perceptions about aging in a new survey from the Pew Research Center. The disparities between what younger people expect will happen as they age, and what really happens, are stark.

Good memory, good health, good sex. It's enough to make the grandkids cringe!

Adults age 18 to 64 were asked what they expect will happen when they get old. Those 65 and older were asked what actually has happened to them. The results (18-64 / 65 and older):

Suffer memory loss (57 percent think they'll suffer it / 25 percent do)Not be able to drive: 45 percent / 14 percentSuffer a serious illness: 42 percent / 21 percentNot be sexually active: 34 percent / 21 percentThat means 79 percent of seniors are having sex, contrary to what they probably expected would be the case when they were younger. A separate study in the New England Journal of Medicine in 2007 found that more than half of 75- to 85-year-olds reported having sex at least twice a month.

And the vast majority of them can drive home afterward, if necessary.

If you think all this has got to make older people happy, you're probably right.

The survey fits with other studies that have shown Americans grow happier as they age. Being male or Republican doesn't hurt, other research finds.

Previous research has shown that happiness in old age depends not just on health and reality, but on attitude. One study found that people age 60 to 98 who had dealt with cancer, heart disease, diabetes, mental health conditions or a range of other problems were surprisingly happy.

Things indeed get worse for those who live beyond age 80, the new Pew Center survey found. About 41 percent of respondents age 85 and older say they've experiences some memory loss and 30 percent are often sad or depressed.

Still, even really old age doesn't sound as bad, on average, as younger folks expect. A mere 1 percent of those 85 and older said their lives have turned out worse than they expected.

The survey, released today and available here, involved 2,969 U.S. adults.

5 Keys to Happiness Happiness Is ... Being Old, Male and RepublicanMore Happy NewsIn The Water Cooler, Imaginova's Editorial Director Robert Roy Britt looks at what people are talking about in the world of science and beyond. Find more in the archives and on Twitter.