I guess that would depend on their maturity levels. You could always open up a custodial account and then once they seem interested, start gradually introducing them to the market by letting them choose companies that they're interested in for their age groups.
In Peter Lynch's book, "Beating the Street," he proved that a class room of 12 yr olds could pick stocks that outperformed those that the top analysts recommended!
The class was broken down into teams and given $250,000 worth of theoretical money and told to choose 10 stocks for their portfolios. The only requirement was that they were able to explain what the company does. It was a competition. The team that won was the team that found a new company that was making colored pens and highlighters. They chose the company because it was something that interested them, was only $5 a share and they realized it had no long term debt! (now how many grown ups do you think do this much research?) lol
I reward my kids with Sharebuilder gift bucks for good report cards and let them choose what they want to buy with it. So far, they're doing better than me! lol