Droppy eyelids or Ptosis is usually repaired in childhood if it is severe and obstructs vision. If mild, it can be repaired either in later childhood or early adulthood.
Q&A below is extracted from: http://sg.theasianparent.com/good-eye-practices/What are the risk factors to consider for an operation on droopy eyelids for a 15-month-old?
As in all surgeries, droopy eyelids surgery carries risk of infection. In some cases, it may result in impaired lid functions. (e.g. cannot close the eye completely). If the droopy lid covers the pupil, that eye may develop amblyopia (lazy). If left untreated, the child may lose vision permanently. It is advisable to see the ophthalmologist soon and surgery may be warranted to prevent/cure amblyopia.