You can't know if you have a vocation until you enter the seminary, so if you are mature enough then you could enter a seminary when you are 18.

If you came late to tradition or just recently found a spiritual director, you may want to get a college degree (if you are already in college, or planning on going into one) first, just in case you are not called to the priesthood you have a degree to fall back onto when you return to the world. Going to college and maturing spiritually under a priest's guidance are not mutually exclusive. For some, it may be a good experience; for others, it may be better to enter the seminary immediately.