There is no specific age. The idea is that since it is an Ashkenazic custom not to eat egg-matza on Pesach, it is not permitted unless there is a special need. For example if an elderly person has digestive problems and egg-matzos are easier to digest. In each case of need a local Orthodox rabbi should be asked if it is okay for that specific person due to special need and circumstances.
May you have a happy. healthy and kosher Pesach!