The concept of FIRE — financial independence, retire early — is a growing movement that has ignited among young people. There is a litany of blogs, podcasts and Facebook groups dedicated to helping people retire early, sometimes even in their 30s or 40s, by practicing habits like extreme frugality, saving or investment.
But star of ABC's "Shark Tank" and self-made business mogul Kevin O'Leary isn't sold.
"This whole idea of financial independence, retire early doesn't work. Let me tell you why," O'Leary tells CNBC Make It. "It happened to me."
O'Leary sold his first company, Softkey Software Products (which was later re-named The Learning Company), to the Mattel Toy Company with nine other co-founders for $4.2 billion in 1999.
"I achieved great liquidity and I thought to myself, 'hey I'm 36, I can retire now,'" O'Leary recalls. "I retired for three years. I was bored out of my mind."
"Working is not just about money. People don't understand this very often, until they stop working," O'Leary says. "Work defines who you are. It provides a place where you're social with people, it gives you interaction with people all day long in an interesting way. It even helps you live longer and is very, very good for brain health."
Indeed, research has found that there are a number of health benefits linked to older-age retirement, including a decreased risk of dementia. And a paper published by the National Bureau of Economic Research found that social contacts are a side effect of employment that keeps workers mentally agile, and that even disliked colleagues and bad bosses are better than social isolation because they help keep the mind healthy and active.
It is worth noting, though, that not all FIRE participants necessarily strive for traditional retirement.
A previous CNBC report, for example, featured Tanja Hester and her husband. Though they quit their 9-to-5 jobs, for them FIRE was more about "[defining] for ourselves how much of a role we want work to play in our lives. ...I do a lot of stuff that looks like work but it's all projects that I feel passionate about," Hester said.
The financial freedom part of the equation is something that O'Leary has talked about in the past.
"I don't need more money," he adds. "I just want to do the things in my life that I want to do, and money allows me to do that," O'Leary previously told CNBC Make It. "I tell every entrepreneur, 'Don't focus on the cash. It's nothing about greed. It's all about personal freedom.'"
O'Leary has achieved financial success, but he still works hard, spearheading the O'Leary Financial Group, O'Leary Fine Wines and starring on "Shark Tank."
"I don't know where I'm going after I'm dead," O'Leary says. "But I'll be working when I get there too."
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Disclosure: CNBC owns the exclusive off-network cable rights to ABC's "Shark Tank."