Cinemark review: poorly treated by manager at location

Yesterday, Sept. 5th, I went to this location to see the new movie IT: chapter 2 with my brothers. We got our tickets in advance- I am a member of the movie club and have always loved this location. & It is always my go to spot.
When we got there and I received my tickets at the front, we went inside and had our tickets looked at by the worker in there. A man, who's name was Adam I believe, came over as we talked to the employee at the ticket counter and asked to see my ID. I handed it over, showing that I was over 21, and was then asked for my brothers ID. I explained that they were 16, didn't have an ID, and were going to be accompanied by me.
He then shook his head and told me that since I wasn't their parent that they weren't allowed In with me. I explained that I was over 21 and accompanying them and was their guardian, and was told no once more. When I further questioned why we were being denied, I was told that it is a Florida state law that kids under 17 have to have a parent with them to see a rated R movie- being their sister and over 21 it didn't matter. I knew this was false, because I have always taken my brothers to see rated R movies, because our parents never cared to watch scary movies. I've always bought their movie tickets and showed my ID at other theatres and have never had an issue.
When I once again questioned what the man said was the state law and I, once again, tried to explain that I was their guardian and over the age of 21, he walked away from me and said he wasn't arguing anymore.
This is highly unprofessional of someone that is a manager of an establishment and has to deal with customer service. I believe that if I appeared older (I am 23 but do look rather young) that we would've been treated much differently.
I asked to speak to someone else, where I was then told that it wasn't a state law but that specific theatres policy. I understand that each theatre is going to have their own regulations sometimes and that's understandable- but the way that we were treated and that information was relayed was unprofessional and disrespectful. I was told repeatedly that it was a state law- when in fact, it wasn't, and each time I questioned him he would become irritated and walk away from me.
I've always had a pleasant experience at this theatre and it's sad that this had to happen the way it did. If it is a theatre policy, I hope that next time that that information is relayed respectfully instead of lying to customers and telling them that there is a state law in place which doesn't exist.
I am now going to have to cancel my movie club membership over this (which my whole family participates in) and the customer service at this theatre is definitely at fault for that- not the theatres policy.