Does FaceID improve over time?
As the system is used, it learns about typical variations in a user's
  appearance, and will adjust its registered face data to match aging,
  facial hair growth, and other changes using the Neural Engine. The
  system will recognize a face wearing hats, scarves, glasses, many
  sunglasses,9 facial hair or makeup.10


https://en.wikipedia.org/wiki/Face_ID#Technology