Seminary education as we know it is a relatively recent phenomenon. It was only at the Council of Trent, called by the Catholic magesterium primarily to fight nascent Protestantism, that the seminary was invented. In the 23rd session, on July 15, 1546, the Council decreed that seminaries be established and start admitting boys as young as 12:
Besides the elements of a liberal education, the students are to be given professional knowledge to enable them to preach, to conduct Divine worship, and to administer the sacraments.
Not long after, the young Protestants followed suit, and since then we’ve had residential seminaries — not unlike other universities and graduate schools.
But for the 15 centuries prior to the Council of Trent, clergy were trained otherwise. How?, you ask. I’ll tell you: