Why Kevin O'Leary says every teen should invest part of their summer job income

For teens and college-aged young adults who are working this summer, it's not too early to start thinking about retirement.

That's according to "Shark Tank" judge and O'Shares ETFs chairman Kevin O'Leary, who says that even before he became Mr. Wonderful, he always had an eye on his financial future.

"Even at the age of 18, I was learning how to invest," O'Leary tells CNBC make It. "It's a great thing to do."

No matter how much money he was making — O'Leary says he laid bricks, collected garbage, washed trucks and worked in a warehouse to earn $10 to $12 per hour — the now-67-year-old entrepreneur was diligent in putting 10% to 15% of his earnings into index funds like the S&P 500.

"If you start doing it when you're 18, 19, 20, 21, 22, by the time you're 65... you'd have about a million and a half in the bank, which is what you want when you're going into retirement," he says. "When you start doing that at an early age, it's a big upside for you down the road."

Having adequate retirement savings is crucial if you want to live off of it for the rest of your life.

To get there, "the key is discipline," O'Leary says. "You're going to get paid something, and you should take a portion of that and invest it."

Legendary investors like Warren Buffett have also long touted index funds, which hold every stock in an index, as the best way for most people to invest. Buffett previously told CNBC that for people looking to build their retirement savings, diversified index funds make "the most sense practically all of the time."

"Consistently buy an S&P 500 low-cost index fund," Buffett said in 2017. "Keep buying it through thick and thin, and especially through thin."

Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank."

Sign up now: Get smarter about your money and career with our weekly newsletter

Don't miss: Warren Buffett says 'monkeys' could do as good a job investing as Wall Street financial advisors