I think a lot of people infantilize the youth. In fact, the word “teenager“ was not used as a term for people aged 13-19 until after WWII. Before then, young people were referred to as, well, young people, along with adolescents and youth. They were seen as inexperienced, but fully capable of doing adult activities and performing mature tasks.
One of these things was marriage. It was not uncommon back in the day for sixteen year olds to drop out of high school, find a job, and then marry/raise a family. Instead of being told “you are too young!” they would receive support from their neighbors and family as they started their adult life.
Today, however, we are living in a culture where teenagers are raised to act like kids yet be skilled enough to ace through advanced classes. “You can’t go to the bathroom without the teacher’s permission, but so help me if you don’t try your hardest to pass your AP biology class!!!!”
Instead of adolescence being a time in life to prove ourselves and earn the title of “hardworking adult” by doing adult things, it’s more of an awkward phase people are stumbling through while being babied and ridiculed at the same time.
What does this have to do with your question, then?
Well, it all has to do with how we now view younger people. Even though those aged 18-21 are legal adults who can smoke and/or drink along with buy a car and go to school and do other things, they are instead seen as rapscallions who are more or less immature adults. And it doesn’t help that we are RAISING our youth this way; any youth who are very mature like their ancestors were and are raised as equals have to fight against the grain to start their lives.
This includes marriage.
If I could have had my way, I would have dropped out of my senior year of high school and married at 17. My body had matured and was ready for babies, I was in love with a childhood friend whom I knew and trusted, and high school was not offering me what I wanted out of life.
If I had my way, I would have dropped out, gotten married, finished my GED online, and then attended a technical school (maybe massage therapy!) and started out my marriage this way while expecting our first little one.
I say “Is it wise to marry young?” is not the right question. Instead we should ask, “Is it wise to treat our young as incapable of making mature choices in life, and force our mature youth to follow the false ideal of teenagehood?”
Yes, people can marry young. I’ve done it, other people have done it: the success of marriage is weighed not by age, but by the couple’s relationship with God, their devotion to their vocation, and by carefully praying and getting to know the other person in a sufficient manner before tying the knot.