I can only answer for Texas. Here you must be 21 to become an LEO. In the meantime getting college or if you are willing join the military. Get their good benefits when you leave. The military pays you when you go through the police academy. Here in Fort Worth the city also pays you. Lots of good benefits from the military that you can take advantage of. The police academy is college type classes. Experience has shown that those with college experience or military do better in our academy. Maturity through both avenues is a big plus to getting on here with Fort Worth.
This is something that varies from state to state. Generally, the more higher ranking the position is, the more likely that the position will require a post-high school degree of some kind. That is, State Troopers or State Police likely require more education than perhaps your local police force (assuming you do not live in a huge urban area). I would encourage you to explore the different areas and levels of law enforcement in your area and identify the qualifications for those positions. An associate's degree in criminal justice, for example, will make you more competitive at the police academy while allowing you to explore all the different specialities that interest you and match with your specific skills.
To repeat everyone else's fine answers, it depends on where you live. Where I am, you should be 21 and should have some college experience IF you don't go through a cadet program.
In most if not all states you have to be 21 to be a sworn officer. Being an explorer is a great start to gain some experience. Some departments hire students for part time duties, which is also good. I would suggest speaking with your local agencies to see what they offer, take some college classes, and/or the military to gain life experience.