Harboring a minor without knowledge

I have a college course, and in my journey to becoming a lawyer an interesting question came up in my essay. Lets say someone is between the ages of 17 and 18, maybe within 2-3 months of becoming 18. Even in the early stages of 17 they may be abled to pass as 18. So, if you were to harbor a minor in running away how would you know in this case? If you harbored one without knowledge can you still be charged? It also brings up an interesting issue since if you can't be charged people can use it as a false defense, but then it furthermore really screws over those that truly weren't aware. It's a question that I've found nowhere on the internet and has utterly confounded me.