Update for any young applicants reading this thread
I applied to biomedical PhD programs as a 20 year old college senior and got interviews at nine out of the ten schools I applied to. All were top programs at prestigious institutions. Age wasn't an issue at all during the interviews, professors were happy to see young students excited about science and eager to start. I've only worked in one lab and never had anything published, but most professors were perfectly okay with that. Seemed like you're fine on research experience if you have proven that you are comfortable and confident in the laboratory environment, and that you are on top of things with your own research project. I've never done most of the techniques commonly used in the labs of professors I was interviewing with, but I wasn't even questioned about it. No one seemed to think I would have trouble getting trained or catching up.
I don't think I'm at a huge disadvantage being young, and I'm glad most of the professors and admissions committees agree. The Ivies were the most friendly, with a lot of professors telling me how happy they were to see young students like me. Stanford was the worst. Lot of rude professors, directed question-and-answer style interviews instead of conversations, and all the current students were old.