Mary’s age is not known, but it is likely that according to Jewish custom of her time, she was a teenager. Not a tween, but a teenager.
In those days, and for many decades thereafter, women’s main job was to bear children. Women were generally married in their teens, at the point when their bodies had developed enough to carry and bear a child. Men generally sought young virgin brides because those women had the longest time window of childbearing and had obviously not had any sort of relations with any other men. Once married, the women hopefully would bear many children before she herself died, which often happened in her 30s or 40s if not younger.
Also, people matured a lot younger in those days because they had to. Children generally were expected to take on responsibility and start earning money for their family at a younger age; many children were also orphaned or put into some kind of slavery or servitude at a young age. Like I said, people often died by their 30s or 40s so you couldn’t exactly be waiting till you were 25 to start an adult life.
Even in the 1800s and early 1900s, young children had a lot of responsibility by age 11 or 12 and often were married themselves by their mid-teens. Society encouraged them to grow up fast, most of them did not go to college and many didn’t even go to high school, and there were a lot of opportunities open to them, like jobs and marriage, that wouldn’t be open to a 16-year-old today.
Nowadays society encourages young people to stay immature and not take responsibility for themselves, so we end up with 25-year-olds still dependent on their parents.