I'm well aware that children can, on occasion, be incredibly insightful. But it's unlikely that you're going to be able to compile a slew of sagacious truths from children, teenagers and twenty-somethings. And considering the fact that you only plan to include 100 pieces of advice, the book isn't going to be appealing to people who are actually looking for something engaging to read. After half a dozen pedestrian platitudes and corny cliches, most people would get bored just flipping through it right there in the bookshop. Few would be inclined to fork over money to purchase it as the minimal content doesn't lend itself well to re-reads. I'm not sure how many people there are out there who are thinking: "You know what my library is lacking? Books chock full of life advice from 11 and 12 year olds." And of course, you'll have to gather the advice first, and it'll have to be perceptive - stuff that isn't all over the internet already. And you'll have to interview people to acquire it. And people will expect to be paid if you plan to use their ideas in a book, so unless you're willing to spend a great deal of time interviewing people and sifting through mounds of useless nonsense in an effort to find 100 tidbits that might fit the bill, and unless you're willing to pay people so that you might feature their advice in your book, I'd say it's a waste of time. But if you really want to go through all that, then have at it.