When Doreen O'Brien celebrates her 50th birthday on New Year's Eve, the ball will drop on an era in America.

As one of the last baby boomers to cross the threshold into official old age, O'Brien brings up the statistical rear of the nation's postwar spike in fertility, which lasted from 1946 to 1964.

"It's a different world than when my grandparents were 50 — because when you turned 62, you were done," said O'Brien, a Minneapolis financial consultant and mother of two. "Now, you can be engaged as long as you're able. Fifty is the new 40."

The story of the tail of the boom is still being written. O'Brien celebrates this milestone birthday at a time when people are living longer and putting off retirement, either by choice or necessity.

Unlike the first wave, which began hitting retirement age by the tens of thousands in 2011, many of the younger group are in good health, raising families and still in line for top jobs. Even if their retirement savings aren't the greatest, they potentially have time to make it up.

Though they grew up in the shadow of the first wave of boomers, far more children were born during the second half of the boom than the first.

"This age group of 50-year-olds is important just because of their sheer size," said state demographer Susan Brower, who noted that there are more Minnesotans in their 50s than any other age group. "For the state, the implications hinge on what the younger baby boomers do as they move into retirement ages."

The question of whether the current crop of 50-year-olds should even be lumped into the baby boom generation is a live one.

The nearly two-decade span of the baby boom generation (so named in a 1964 Washington Post story) was defined by census trackers based on birthrate.

The younger set's political, social and economic reference points are radically different from those of the war protesting, civil-rights marching, flower-power espousing children of the 1960s. This group was shaped by Nixon's resignation, video games, punk rock and AIDS.

The boomer 'litmus test'

O'Brien's world was rocked by the Iranian hostage crisis when she was in high school in the early 1980s. She grew up playing "Pong" and went to college when it was affordable. The Beatles released their first U.S. album the year she was born. Her musical touchstones center more on big-hair bands and MTV, and she vividly remembers her first concert, the Bay City Rollers at Orchestra Hall.

Culturally, O'Brien feels more aligned with Generation X-ers, and many historians and economists would agree. Humorist P.J. O'Rourke wondered in an AARP article whether late boomers "feel as if they're at the end of the high school cafeteria line with nothing left but cold pizza muffins."

The boomer "litmus test," says well-known generational historian Neil Howe, is whether you can remember when Kennedy was assassinated. Others draw a line on either side of the Vietnam War. Many demographers merely break up the massive cohort, now about 76 million strong, into two halves: those in their 50s and those in their 60s.

No matter where the generational gate gets dropped, the current set of assumptions about aging won't apply to today's 50-somethings.

Like a lot of younger baby boomers, O'Brien waited until her mid-30s to get married and start a family, benefiting from newly opened career paths and the widespread availability of the Pill.

More young adults live with their parents these days, which may delay when O'Brien and other late boomers become empty nesters. This will influence their timing and choices about downsizing or relocating.

"You're talking about a lot of change in households within the next 10 or so years," Brower said.

No retirement in sight

But the big question remains how long these younger boomers will work.

By the time O'Brien hits retirement age, 80 million Americans could be tapping into Social Security and Medicare. Financial, health care and lifestyle decisions she and younger boomers make between now and then could dictate how well the United States navigates the coming age quake.

If today's young boomers follow current patterns and delay retirement, more older adults will be working. The ripple effect in society could be broad, as one's work life affects spending, saving and leisure time.

O'Brien envisions a work life that goes on as long as she's able, be it full time, part time or taking on three-month projects.

"People my age don't even think about a traditional retirement," she said. "It's not necessarily bad. I see us as more engaged."

O'Brien acknowledged, however, that she can't keep up her current pace through middle age:

"When I realized that I was the last baby boomer, it kind of takes the edge off it."

Jackie Crosby • 612-673-7335