I'm not the slightest bit confused. The original poster doesn't allow guests under the age of 18 under any circumstances.
The original poster appears to be confused as he/she says that someone on Facebook said that he/she was discriminating and went on to ask if he/she is breaking any Airbnb rules. As I said, he/she is definitely discriminating by the dictionary definition of the word, but is not breaking the law, nor Airbnb's rules. I think that everyone is getting their knickers in a bunch because they think discriminating always carries a negative connotation. Once again look at the dictionary definition.
I don't discriminate among Airbnb guests. I take everyone as I have Instant Book enabled with no filters.
In other areas of my life, I am very discriminatory. I only have rescue pets, none from breeders. I don't watch violent movies. I don't eat meat or poultry.