A 5 year age difference isn’t the problem it’s being several developmental and societal milestones apart. That’s why it’s gross, and not just in the U.S. but in many countries.
A 5 year age difference between a 25year old and a 30 year old is benign a 3 year age difference between a 13 year old and a 16 year old is bad.
In your case developmentally it should be obvious in terms of both physical/mental development but unless you missed numerous key life events you should be at a very different stage in your life. As she is entering the workforce/post secondary education you should be leaving post secondary/ being at an intermediate level in your career path. While she most likely is still firmly living at home and has others taking care of her you should be able to be fully independent even if you are still living at home, while she can’t legally buy a drink you probably have a substantial alcohol tolerance.
Like sure there are countries where adult men marry literal children 10-13 but we don’t call those countries open minded
In Romania the legal drinking age is 18 but regardless if we say it’s commonplace to drink at 17, she just entered that period of time whereas you have been doing it for 5 years.
Your parents may support you financially but you are expected to learn life skills in those 5 years, also you should be finishing school where you have learned everything you need to further your career and she is just entering it does that not seem strange to you?
Really the biggest problem is that there is a power dynamic between you, you are more educated, more developed, more experienced, when a relationship should be between two equals. What makes it so much worse at this age is how quickly you hit milestones and how quickly you can completely eclipse the other person