There are still college seminaries in the U.S., not as many as there once were. Many college seminaries also conduct what are known as pre-theology programs, basically one or two year programs where men with bachelor’s degrees in other subjects complete the equivalent of a B.A. in philosophy. Many pre-theo programs are attached to major seminaries, so that the seminarian stays at the same seminary for a total of 5-6 years until completing the Master of Divinity degree and being ordained.
I agree that a great deal of thought and prayer should go into the selection of what I think of as a “vocation path for discernment.” I learned the hard way at age 17 when I sort of stumbled into the seminary system, and now much later I have been discerning a vocation much more consciously.
As this is a liturgy thread, the first thing I would advise is trying to attend a few liturgies at your prospective seminary. For me the biggest shock of my life was going to the seminary full of devotion and then being forced to stand during the Eucharistic prayer, deal with crumbling Eucharistic particles in the carpet, etc.
Having said that, know that most dioceses and religious orders have a narrow range of seminaries they use, and that religious orders tend to use theological unions or houses of studies attached to universities for academic formation while spiritual formation is held within the order. Once you are decided on a given diocese or order, you most likely will not have a lot of choice about what seminary you attend, especially if you discern a vocation to an order.
Most college seminaries these days are a mix of college-age men and thirtysomethings doing the pre-theology program. The emphasis is on discerning no matter what your age more than on preparation or certitude that you are called to be a priest. Make a thoughtful, prayerful choice so that you can take your philosophy studies in a good spirit and (God willing) go on to become a good priest.