Before age 3, children’s questions are generally incomplete, depending on words like “where” or “what” and a rising tone at the end of the sentence. Around age 3, children begin to use more complete constructions. For example, a younger child might ask “Where doggy?” while a child who is older may ask “Where did doggy go?” or “Why it’s not working?”
Hoff-Ginsberg, E. (1997). Language development. Pacific Grove, CA: Brooks/ Cole.