How old were you when you started working full-time? Chances are it was sometime in your early 20s. Or maybe your late 20s if, say, you went to medical school. But that's all wrong, according to psychologist Laura Carstensen, director of the Stanford Center on Longevity. Instead, she argues, we should all be beginning our full-time careers when we're about 40.

You may find that shocking, especially in a society (and on a website) that idolizes the young and successful, the younger the better. But Carstensen actually has some very good arguments for her point of view:

1. People wouldn't have to work their hardest when they're also raising small children.

In the current system, people get an entry level job right after college or other training, usually when they're between 20 and 25. Until recently, that was also the age range during which they were most likely to be starting a family. The average age to have a first child in the US has been rising. It's now around 30 for men and around 28 for women. There are many reasons for this development, including the growing burden of student debt. But another factor may be the long hours and intense dedication you often need to put in when starting out in a career. It's tough to make that work when you're doing 2 am feedings. Even those who don't start having children till their early 30s should have those children in school by the time they're 40.

2. There would be less burnout and frustration.

In our current model, "You never get a break. You never get to step out. You never get to refresh. We go at this unsustainable pace, and then pull the plug," Carstensen told Quartz. That system doesn't work, she says, "because it fails to recognize all the other demands on our time."

Or our dreams. In the classic Cary Grant-Katharine Hepburn film Holiday, Grant plays a young man who's been working hard since he was a child. He's made a success of himself so far--and now he intends to take a few years off. "I want to save part of my life for myself," he explains. "There's a catch to it though, it's gotta be part of the young part. You know, retire young, work old, come back and work when I know what I'm working for."

Young people who haven't take the time to do that, to see the world and try to figure out their place in it, are likely as not to embark on careers that aren't right for them. That's one explanation for the frequent phenomenon of people getting law degrees but then choosing not to practice law. Taking some time to explore the world and figure out what kind of career is right for you increases the chances that you'll be happy with that career, and want to stick with it.

3. It would accommodate longer lifespans.

Women are living to an average age of 85, and men to an average age of 82. Given those demographics, retiring in your early 60s, or even at the current "full" retirement age of 66 seems both unnecessary and possibly impractical, given how many years our retirement savings will need to last us. If you're going to work until you're 75, which is becoming less uncommon, then starting at 40 will still give you the same 35-year career you would have if you started at 25 and retired at 60. It's worth noting that, your Social Security retirement benefits are calculated on your 35 highest-earning years.

4. It would eliminate the "square wave."

Social scientists refer to our current career paths as a square wave. You go from not working at all (or perhaps a part-time job in college or high school) to full-time employment. You do that for 40 years or so, and then when you retire, you stop as abruptly as you began. In Carstensen's model, instead of starting out full-time, people in their 20s and 30s could continue their educations for longer, or work part-time or as apprentices, before reaching full-time employment around 40. Then, as they age, they could retreat from the workforce gradually, working part-time or on a consulting basis in their late 60s and 70s, and retiring fully at around 80. 

This would have several benefits for seniors. They wouldn't have the abrupt drop in self-worth that comes with having a job one day and not having it the next, or the common problem of not knowing what to do with their time. They'd also be less likely to run out of cash if they live into their 80s or 90s, or like my stepfather, into their 100s. Working later in life can lead to greater mental agility as well. 

There are advantages, too, from the employer's viewpoint. The current wave of Baby Boomer retirements has many worried about the institutional knowledge walking out the door with these departing seniors. Given that concern and the current labor shortage, many employers are inviting retirees to work on a consulting or part-time basis for a few more years.

It won't happen tomorrow.

Other than that, there are many reasons why Carstensen's vision for a redesigned work life won't come true anytime soon. To begin with, 40-year-olds applying for their first full-time jobs are likely to face some skepticism from prospective employers. And the economics of paying off student loans while starting a family and perhaps purchasing a first home will make it tough for most 25-to-40-year olds to get by on less than a full-time salary. Our youth-loving culture is also a problem: In many professions, people are considered to be on the decline after 40.

It's a shame, though. Because ramping up slowly to full-time employment at 40 would help many people have better work-life balance and allow them to spend those all-important early years with their kids. It would also give them the chance to get a better perspective on the world and themselves. And then, like Cary Grant's character in Holiday, they could work when they really knew what they were working for.