Teenage bodies are constantly growing and changing and during this time, eyesight is liable to change seemingly overnight. You should schedule regular eye exams for your child or teenager as eyesight can change until he is in his mid twenties.
If your teen has no pre-existing eye condition such as myopia or hyperopia and no family history of eye conditions then a visit to the opticians every year or two is sufficient.
However, if your teen has previously been diagnosed with an eye condition then it is advisable to schedule more frequent eye tests.