Avaz can be used by anyone with complex communication needs, regardless of the age.
The app may appear like it's designed for children because of the stick symbols used for depicting a word - which makes it look more child-friendly. The picture vocabulary has certain folders that are more relevant for children, e.g. Toys, Games etc. But you can certainly customize the vocabulary to suit an adult's needs. For example, the folder of Toys or Games can be replaced by Activities or Hobbies that the adult is interested in.