For children: According to the American Optometric Association (AOA), children generally should have their first eye exam at six months of age, another exam at age three and again at the start of school. Risk-free children should then continue to have their eyes examined every two years until age 18. For adult: To maintain a lifetime of healthy vision, the AOA recommends a comprehensive eye exam every two years for adults ages 18 to 60, and annual exams for seniors age 61 and older. Source:http://www.allaboutvision.com/eye-exam/preparing.htm
For children, it is good to have their eyes checked every year because the eye conditions of children are unstable and always changing. For adults who are under 60 and have perfect vision, it is still good to have their eyes checked at least every two years. If you are over 60, you'd better have an eye exam once a year.