Attention, parents: "Money doesn't grow on trees" has a shelf life. Be sure to check this beloved nugget's expiration date before you crack it open and serve it to your offspring.

A few guidelines: When your little one can still fit into the grocery cart, "money doesn't grow on trees" is at its full potency. Its effectiveness starts to wane in the pre-teens, particularly when you use it to pry Junior from the video game aisle. By the time your little lovelies head to college, the game is over.

Spendthrift U
Just try to use that line on the more than 17 million students currently settling into college life. That is, if you can get a word in while they snap up free T-shirts and pizza and credit card applications. Before they have a credit history or even a job, they're plied with access to spending money -- money that they haven't yet earned and that's not yet in their checking accounts.

Here's how one college administrator described the scene: "Like sharks at a feeding frenzy, banks give away cheap, huge chocolate bars and stuffed animals as come-ons to students to get them to sign up for their cards."

Her college eventually banned credit card pushers from courting customers on campus. But that doesn't mean the temptation isn't still there.

Despite what they say, everyone else is not doing it
Oh, yes, they are. (We're still talking about credit cards, by the way.) A 2003 study by student-loan lender Nellie Mae found that 54% of freshmen have a credit card. By the time they are sophomores, 92% of co-eds are packing plastic.

The kids are using them, too. According to Nellie Mae, undergraduates carry an average credit card balance of $2,327, with 21% burdened by balances between $3,000 and $7,000.

As any adult who has learned this lesson the hard way will tell you, credit cards are a difficult habit to kick. Here's a firsthand tale from our own Tim Beyers, who paid off $45,000 in the late 1990s only to rack up another $56,000. (Granted, the majority of the debt is due to a home equity line of credit.)

But it's not just individuals who have a hard time fighting temptation.

Just say no
Turning the other cheek is a challenge for even the strongest-willed student -- particularly when the adults tasked with their care and handling seem to have dropped the word "no" from their vocabulary.

The University of California, Los Angeles, is one of 700 higher-learning institutions that said "yes" to Bank of America's (NYSE:BAC) request to peddle credit cards on campus. And according to a recent article in USA Today, Citigroup (NYSE:C) has dibs on co-eds at more than 200 campuses, and the University of Tennessee will pocket $10 million over seven years by allowing JPMorgan Chase (NYSE:JPM) to hawk plastic to its students and alumni.

As far as restrictions go, they're pretty lame. For example, at the University of Central Florida, Bank of America is allowed to market credit cards only to students at its student union. On Wednesdays.

The Foolish bottom line
Getting kidults to pay attention to some words of wisdom about money may seem like a daunting task. Instead of preaching, try playing into their newfound freedom. Tell them to:

  • Question authority. As long as Junior's learning to question authority, here are a few debating points he can use to take on the lending establishment.
  • Kick some serious butt. When fewer than 20% of workers age 21 to 24 choose to sock away even one dime in their work retirement plans (that's more than twice the rate of non-participation of the baby boomer generation), getting a leg up on your peers should be no problem. Employer-sponsored retirement plans are one place where money actually does grow on trees in the form of an "employer match." Here's a cheat sheet to help them get ahead.
  • Ignore your advice. That's right, there are some parental fibs that can handicap your kids if they carry them into adulthood. Here are five white lies that may still be doing more harm than good.

Consider giving your kids a crash course in credit management, Cash Flow 201, investing basics, and managing your money in the real world with a free 30-day trial of The Motley Fool's GreenLight service. They'll get step-by-step instruction on tackling life's big and small financial matters and an all-access pass to the members-only discussion boards -- like a friendly study hall where talking openly is encouraged. Take a no-obligation trial to start getting ahead of the class financially.

Dayana Yochim is the co-advisor of GreenLight and a debt-free graduate of the University of Kansas. She owns none of the companies mentioned in this article. Bank of America and JPMorgan Chase are Motley Fool Income Investor recommendations. The Fool's disclosure policy graduated summa cum laude.