There is no one-size-fits-all answer to this question. Instead, it helps to keep in mind that medical school admissions decisions are based on evidence the helps them understand:
Your capacity as a student and a scientist, including (but not limited to) your grades (especially in pre-medical science courses), your MCAT score, and to a certain extent your participation in research
The experiences you've chosen to learn more about health, medicine, your responsibilities toward others, and yourself (and of course what you've actually learned from all of them)
The personal qualities they have determined as essential to being a good doctor.
Applicants that provide substantive amounts of evidence for these key areas are more likely to be successful. It takes time to build that evidence, which is one of the main reasons that the average age of medical school matriculants hovers between 24 and 25 years old.
Regardless of how old you are, once you've determined you're ready to apply, most medical schools use some version of rolling admissions so it's best to apply early in the application cycle. The best situation involves you submitting your primary application and your MCAT score by the end of June. Beginning your application late in the summer can cause you problems with some--perhaps many--medical schools.
