When a person on the spectrum communicates something to you, whether it be through speaking, writing, or picture exchange, please listen to them and please reply. Because words don't come naturally to many autistic people, and the words they have gifted you with are valuable. Show them that you see their value by answering back, no matter their age and no matter how grammatically incorrect that sentence may have been. Because that "How are you?" or "Hello!" may seem simple to you, but for an autistic person who struggles with verbal communication (which not all do, by the way), it's a whole lot more than that.