10 and 15 are great ages for geocaching. Recognize that, especially at 10, you're committing the parents to go along for safety, but that's part of what I love about geocaching, going with my kids. I think just about any age can be good for geocaching. It's more a matter of if the activity appeals to an individual. There are some folks out there who don't understand what the fun is, wandering around looking for trinkets. I don't know what's wrong with them, but they are out there. :-)