You are not eligible for Tricare unless you serve 20 years.
One caveat: You get Tricare (for life) only if you retire from active duty after 20 years. If you retire from the reserves with 20 years of service, then you get Tricare (you pay monthly) and you get Tricare for life when you turn 65.
Did I mention the Reservist don’t get retirement pay until they turn 60.