Everything we think we know about nuns and monastic life today was actually created by a lesser known Saint from the 300s- Macrina the Younger.
Prior to Macrina, nuns were expected to be virgins and virtuous from the very beginning. Nunneries would not accept a widowed woman or anyone who might have come into their religious awakening late in life. Macrina actually was a virgin when she joined a convent, but coming from a wealthy family, she was forbidden from taking vows by her father. He betrothed her to a similarly wealthy young man from another prosperous family who passed away before they were actually old enough to marry.
Macrina took advantage of the situation; while her father sought another young suitor for her, she declared herself a "spiritual widow," since she had been betrothed to another. Her father gave into her wishes and she eventually setup a convent of her own. In her thinking, however, a woman (or anyone for that matter) could become a virgin by taking vows and becoming chaste. As long as someone committed to it, one could join her convent.
Eventually, her widowed mother would join Macrina, becoming a nun in Macrina's convent after Macrina's father passed away. Oddly enough, despite her father's reluctance, Macrina's family would have three saints in it- Saint Basil, Saint Gregory of Nyssa and herself. Her brother's hagiography of her life would cement her religious achievements. While she has become a lesser known Saint these days in the western world, she is a hugely popular one in the Eastern Orthodox Church.
