Can you be 55 and go to college if you just have high school diploma? To learn how to be a dermatologist?

Dermatologist requires an MD degree before beginning residency (post-MD clinical training). With 4 year bachelor's + 4 yr medical school before even beginning residency, do you want to try looking for a residency (assuming you are top graduate of a highly rated university AND medical school) when you are already retirement age? Never gonna happen.

The problem is not getting into community college, perhaps university. You're choosing a totally inappropriate career goal at this point in your life.

Of course, but you will never practice as a dermatologist. By the time you graduated and obtained your license you would be nearly 70.

You can go to college no matter how old you are. You may or may not have to take a placement test.