In the modern era, when we think about college, it comes after a young person has completed 12 or 13 years of school - or primary and secondary education. Tertiary education, or higher education, is a third phase or stage. At the same time, college is now generally seen as a necessary step before entering politics, law, or the clergy (the most common professions among the founders.) Both of these features - college as something young people routinely do and something that's needed before entering the white-collar workforce didn't really take shape in the United States until after World War II.

During the pre-colonial and colonial era, there was no formalized sequence of events. A young white man might go a grammar and then high school and then one of the colonial colleges or he might sit for the college entrance exam after working with a tutor for a few years. Harvard accepted students as young as 10 as the main criteria was that they could pay tuition, were of good character (wouldn't embarrass the college), and had sufficient background knowledge to be successful in first year classes.

Another key difference is that the coursework and content at the early colleges wasn't organized around a particular major in the way we think of it today. Instead, students studied a classical, or Latin, curriculum that was focused on learning the things that smart men knew. So, to a certain extent, there was less to learn because the curriculum was fairly tightly focused on a few subjects - Latin, Greek, some math, some science, and logic or rhetoric. This is why there is a certain cadence or similarity to a lot of the writing from the founders - they read a lot of the same texts as they learned to craft arguments.

More here from u/lord_mayor_of_reddit and me about college admissions in the Colonial Era.