What could it mean if you're still afraid of the dark at the age of 21?

Fear: Persistent fears can represent many things, triggered by experiences someone has had, heard about, read about, or seen in dream(s) or movies. If your fear of the dark is causing you discomfort or interfering in your life, it warrants an assessment. Although the source of your fear may be "unknown", which is also scary, usually once the source of fear is identified, treatment can change your future.

Fear: If could simply mean you have conditioned your brain to fear darkness even though your rational brain probably knows your are ok. Of course this fear could point to other difficulties like anxiety, insecurity or fear of separation but not necessarily. You could always consult with a psychologist if you continue to be troubled by this fear. Best wishes.