If I enlisted in the Army at 17, how long would it take for me to be able to apply for SFAS?

I've been figuring it would be a better idea to enlist in the Army at 17 to gain some experience as a Soldier before I'm able to apply for SFAS instead of going the 18x route and waiting three years until I'm 20 to enlist. So, if I enlist at 17 when would I be able to apply for SFAS? Is going this way better than to go the 18x route? Thank you