Baby boomers are fretful about the U.S. economy, leaving cheerier millennials on the hook to keep spending and overall growth on track.

Going back decades, consumer surveys have shown Americans in their 20s and 30s more optimistic about the economy compared with their parents and grandparents. But the generation gap has been unusually wide in recent years in gauges from the Conference Board and University of Michigan. Confidence among consumers under age 35 is back to prerecession levels, while sentiment among people 55 and older remains far lower, deteriorating for more than a year.

The divide, partly driven by greater optimism about income growth among younger households, helps explain why consumer spending decelerated in 2015 and early this year despite low interest rates, cheap gasoline and falling unemployment.

Older Americans pulled back their spending last year and in the first three months of 2016, according to data from Chase credit and debit cards, while younger Americans ramped up outlays.

“All the growth is being driven by young people, and in fact the older people are dragging down growth,” said Diana Farrell, chief executive at J.P. Morgan Chase & Co. Institute, which analyzed the spending data.

Ray Hernandez, 35 years old, and his wife live in Heath, Texas, in the booming Dallas metro area where they bought their first house in early June after a four-month hunt.

“You can tell the economy’s great because every house we tried, we’d get overbid by, like, $40,000 in cash,” he said.

He admitted to some nervousness about the future. But Mr. Hernandez, the creative director at a Dallas technology company, said for now, local restaurants are full and “everybody’s out spending money.”

The limited spending power of young Americans will likely struggle to carry economic growth indefinitely.

Many 20- and 30-somethings bear high student-debt burdens that could restrict their ability to buy a home or make other major purchases.

They typically earn and spend less money than middle-age households, according to Labor Department data. And the median net worth for families under 35 was a mere 6% of the median net worth for 55- to 64-year-olds as of 2013, according to the Federal Reserve.

It’s “a little bit more confident consumer, but also a less wealthy consumer,” said Marshal Cohen, chief retail analyst at research firm NPD Group.

Consumer spending is such a dominant force in the U.S. economy that even a modest sustained decline in outlays typically heralds a full-blown recession.

Growth in inflation-adjusted spending has slowed over the past year and a half, expanding at a 1.5% annual rate in the first quarter—the smallest increase in two years and about one-third the pace seen in late 2014, according to Commerce Department data.

Still, that was enough to keep overall output expanding in the face of headwinds including a sharp decline in business investment.

To be sure, the link between consumer sentiment and actual spending has been fuzzy at times. There were signs of a pickup in household outlays this spring even as readings on confidence remained mixed, boosting expectations for overall economic growth in the second quarter.

Still, businesses have been adapting to the shift in spending.

“It used to be the retired, the people who had time and money to spend on this,” said Todd Leff, chief executive at Hand & Stone, a Trevose, Pa.-based chain of massage and facial spas. Now, he said, it’s “stressed-out young professionals and young parents.”

As its customer base has skewed younger over the past five years, Mr. Leff said the company has reoriented. More money goes to advertising on Facebook, he said, and memberships that used to require a yearlong commitment now are available on a month-to-month basis.

“Certainly we want everybody,” he said, but “we see the millennials as the customers of the future.”

Older consumers, for now, are wary about opening their wallets.

The Conference Board’s consumer-confidence index has moved lower since early 2015, depressed by falling confidence among people 55 and older.

Readings for consumers under 35 are choppy from month to month but have remained near prerecession levels. The confidence gap between the under-35 and 55-and-over categories hit a record in June, at roughly three times its average level since 1980.

A somewhat similar pattern has emerged in the University of Michigan’s long-running survey of consumer sentiment. The gap between 18-to-34 year olds and 55-and-older households widened to a record last August and remained larger than normal into the spring.

Richard Curtin, the Michigan survey’s chief economist, said the age gap reflects stronger confidence about wage growth among younger Americans and tends to widen as economic expansions proceed.

Now, seven years after the last recession ended, he said that “older households have become more concerned about the future of the economy,” and so “have acted to increase the proportion of their savings” by putting off spending and borrowing.

That could become a self-fulfilling prophecy, if a pullback in consumer spending leads to a broader economic slowdown.

Spending by seniors has been declining at Checkers and Rally’s brand fast-food restaurants while spending by younger customers has been rising, said Terri Snyder, the chain’s chief marketing officer.

She said young diners are benefiting from minimum-wage increases in many states and cheap gas, but many older Americans are constrained by fixed incomes and rising medical bills.

“While younger people have more disposable income, the seniors probably have the toughest time of anybody right now,” Ms. Snyder said.

Stock-market volatility and global uncertainty in the wake of the U.K. vote to exit the European Union could further damage confidence among older households, which are more likely to have substantial investment portfolios or retirement savings.

And for the next four months, uncertainty at home generated by the 2016 presidential campaign could weigh on consumers, businesses and the economy at large. Political turmoil has a tendency to rattle Americans of all ages. For instance, sentiment and confidence nose-dived amid the debt-ceiling fight of 2011 and the federal-government shutdown in 2013.