I applaud your enthusiasm, but here are some basics for law enforcement.
Most agencies won't hire you for a patrol position until you are at least 21 years old. The state POST (Police Officers Standards and Training) board dictates this, and is not subject to dispute. While the agency can hire you for other civilian positions, a patrol (or even detention) position is out of reach until that 21st birthday.
A general figure for hiring breaks down like this. I'm only going to base this off of 100 applicants (for percentages), but the numbers apply to almost all agencies.
Given that 100 people apply for an open position, the average rate of people passing the minimum required testing process, approximately 10 will pass.
Of those 10 people, maybe 5 will past their background checks and get accepted to an academy. Should they pass the academy, they still must go through a field training program.
So, now you have (if you're lucky) 5 new recruits ready to go out, kick ass and take names. If they make through field training and probation, which usually lasts 6 months to a year, they still have to prove themselves able to handle a street level job and make the smart decisions when faced with life and death circumstances.
You now have a force of the top 5 percent of eligible people, contending for the top spots in any given field within that agency. Most agencies (but not all) will make you wait at least 3 years before you can apply for, test to, and posibly get into any of these specialty positions, ie SWAT, K9, Narcs, CI, etc. And the positions within the agency are usually at, or near, 10 percent of the total work force for that agency.
So that leaves you at 0.5 percent of the original hiring pool making it to those specialty spots in the "elite" fields.
If you're smart enough, strong enough, and able to withstand the grueling hours needed to get this far, keep going! But don't come into it for the money. It isn't there in public service. Satisfaction yes, money no.
Good luck with this.
And yes, I'm one of the "lucky" 0.5%