If someone graduates medical school, you generally call them Doctor, no matter how old they are. It has nothing to do with age, it is a title that you’ve earned by virtue of passing the exams needed.
If someone gets semicha, they’ve earned the right to be called Rabbi. Does that mean you always have to? Probably not, but when using a formal address, you should write their official title.
Whether you have respect for them or not is up to you. If the semicha is from a yeshiva you don’t hold of, then don’t show them the respect to their face. Obviously, don’t use them as your Rav.