You are never too young to discern your vocation (priesthood, married life, single life or consecrated religious life). You must be at least 18 years old to enter seminary. Many men join seminary right out of high school.  

Tell them you believe God is calling you to do this. Reminding them that this is a process of discernment, and that attending seminary does not guarantee that one will become a priest. 

Just because you enter seminary does not mean you are going to be a priest. Seminary is a place to more deeply discern God’s call in your life. Discernment is a process that normally takes years to understand where God is calling you. That may be to the priesthood or not. If you go to seminary and you discern that God is not calling you to the priesthood, it is not a waste of time. Seminary makes everyone a better man, helps them grow closer to Christ, and gives you skills to help spread the Gospel. 

I help my children to listen to God for their vocation. God calls each individual to a vocation—whether it be the priesthood, married life, single life, or consecrated monastic life—in order for them to reach their full potential. The key is seeking “how I can serve in the way God intended” vs “what do I (or my parents) want”. What greater success could there be than saving souls as a priest? 

God calls men with different skills, strengths, capabilities, and backgrounds to be priests. He asks us to give Him our first fruits, not what is undesirable or left over. Jesus himself was “a lamb without blemish or defect”.