From a specifically female point of view, marriage and childbirth were an important aspect in the life of a medieval girl or woman. The risks associated with childbirth, however, were quite high at the time due to a number of factors: age; health and illness; birthing complications; and death.
For many noble-born or royal women, marriage could and often did take place at a young age. There are many instances or very young girls being betrothed and married under the age of 10 years old. This did not necessarily mean that the marriage was consummated. However, there was a perception that once a girl began her period that she was considered to be of marriageable age. And so the male could begin his almighty pursuit for an heir.
So, typically, when did a young medieval girl embark on the road to “womanhood”:
Puberty is the process of change that takes place as you grow up and become physically mature and capable of having children.
Puberty (and thus menstruation / periods) usually takes place between the ages of 10yo and 16yo.
"Most girls start their first periods at about 12 or 13; however some girls may have periods by the age of 8 and still others may not have a period until they are 14 or 15."(Source: About Women's Health).
At the time when we have our first period or "menarche", we are crossing the line from girlhood to womanhood.
Now, marriages of noble and royal women were usually for political and dynastic consideration. So, at what age did a young noblewoman enter into marriage.
It is more common for a young woman to have been married early, though not to have had her first child until she was much older. It is agreed that the most common age for a young woman to have given birth to her first child is from 16yo.
In Italy the average age for marriage was 17; in France it is 16yo; and in England and Germany 18yo was the average age - all for first marriages. (Source: “Medieval Households” by David Herlihy, Harvard University Press, 1985).