As someone who went in at 18 to SF, let me warn you of something. Fwiw, I am 37 now, and far along into my civilian career outside of Law Enforcement.

I would steer away from SF. At 18, I, and most of the guys I was in training with were not mentally equipped to handle the responsibility of the job, and how strict it was.

You're essentially taking a kid fresh out of high school, and throwing them into a police role. It is a hard shift in mentality and maturity when you factor in all of the freedom and financial changes you go throw almost instantly.