That said, this conservative idea that you should meet your future spouse in high school and marry by the time you finish college (at the latest) is crazy. I think people assume that our ancestors all married really young because the most famous historical marriages were all among royalty.
There are a few issues with this. The royal marriage pool was so small that by the time princes and princesses were children, they pretty much knew who their two or three potential mates were. Their royal parents were also almost certainly using them as bargaining chips in treaties or alliances. So their getting married around puberty made sense from a political standpoint, but was as weird as a dog in a tutu to everyone else. Even when they did get hitched that young, couples often didn't live together, and certainly weren't expected to have sex, for many years.
Or, you know, ever.
In general, the marriage age in Western Europe has stayed constant. English records from the 1600s show that brides were usually 23-24 years old and grooms 26-27. When colonists in early America started getting married slightly younger, it was considered odd enough that Benjamin Franklin commented on it. But the marriage age in America soon settled back into the normal pattern, and by 1890, most couples were getting married in their mid-to-late-20s again. While there have been extreme circumstances where kids in their late teens started getting married regularly -- like after the Black Death and WWII killed everybody and people were looking to tie down whoever they could get who still had a pulse -- in general, people have always tried to put off this lifelong commitment until they were actually ready.