From A Wiki of Ice and Fire
A maid or a maiden usually refers to a virgin woman. Maidens may be wedded and bedded... however, even there, many husbands will wait until the bride is fifteen or sixteen before sleeping with them. Very young mothers tend to have significantly higher rates of death in childbirth, which the maesters will have noted. 
A guarantee of a woman's virginity is normally the presence of a maidenhead. Women are generally expected to be virgins on their wedding night, more so for noble women. For example, because Lysa Tully was no longer a virgin her father Lord Hoster Tully found it difficult to arrange a marriage for her to a man of equal rank.
The loss of a noblewoman’s virginity can also become an opportunity for some nobles looking to marry off sons who are not considered good prospects. After many rejections from other Houses Lord Tywin Lannister eventually approached the father of Delena Florent who had given birth to the bastard Edric Storm. Tywin proposed that his dwarf son Tyrion be married to Delena. Her father rejected the proposal, preferring to marry her off to one of his household knights, like Hoster Tully he probably wanted a "whole man" for his daughter.
Virginity can also be held in high esteem by the Dothraki on occasion. It is quite likely that a khaleesi should be a virgin bride. If Daenerys Targaryen’s brother had succeeded in claiming her maidenhead the night before her wedding it would have, according to Illyrio Mopatis, undone years of planning – meaning that Khal Drogo would not have accepted Daenerys if she had not come to him a virgin.
|“||I am still a maid.||”|
|“||Daenerys Targaryen is no maid.||”|
|“||He told me once I was sure to die a maiden." ||”|
|“||A Frey girl, and not of my choosing, she is not even a maiden.||”|
|“||Do you want to die a man-maid?||”|
|“||Myrcella never shed a tear, though it was she who was leaving hearth and home to seal an alliance with her maidenhood.||”|
References and Notes
- ↑ Age of Sexual Relations in Westeros, October 05, 1999 So Spake Martin
- ↑ A Storm of Swords, Chapter 19, Tyrion, p 221.
- ↑ A Dance with Dragons, Chapter 5, Tyrion.
- ↑ 4.0 4.1 A Storm of Swords, Chapter 75, Samwell, p 858.
- ↑ 5.0 5.1 A Dance with Dragons, Chapter 68, The Dragontamer (Quentyn IV).
- ↑ A Storm of Swords, Chapter 68, Sansa, p 775.
- ↑ A Dance with Dragons, Chapter 22, Tyrion.
- ↑ A Feast for Crows, Chapter 7, Cersei.
- ↑ A Feast for Crows, Chapter 13, The Soiled Knight, p 240.