The vagina. What most people see, right from the start, is actually the vulva, the plump folds of skin that include the labia majora (outside lips), labia minora (inside lips), clitoris, and urethra.  The vagina itself is a muscular tube that connects the internal reproductive organs (cervix, uterus and fallopian tubes) to the outside of the body.  Just like other parts of a woman's body, the vagina changes over the course of her lifetime.  

From the time a girl is born until she reaches puberty, the vagina grows proportionally with the rest of the body.  By the time she reaches her full height, her vagina will be about four inches long.  The vulva will be hair-free before puberty and the vagina and labia are kept moist and clean by the mucous and secretions it produces.

A thin layer of skin called the hymen protects the vagina.  For some girls, the hymen remains intact until their first sexual experience.  For many however, it breaks or wears down over time through normal activity, bike riding, and horseback riding or for no apparent reason.  While some cultures still consider the hymen to be an ironclad sign of virginity, women's health specialists know that many girls who are virgins don't have an intact hymen.  

The average age for girls to begin puberty-the transition the body makes from childhood to womanhood-is between nine and 12 years old.  One of the first visible signs that changes are happening is the appearance of pubic hair.  Over the next couple years, as her other reproductive organs (uterus and ovaries) reach maturity, she'll start having periods and ovulating.  The vagina is the exit route for the secretions created by the uterus and cervix in response to normal menstrual cycling.  

During the reproductive years, from about age 13 to 52, the cycle of hormones associated with ovulation, menstruation, and pregnancy, keep the vagina infused with healthy circulation, lubrication, and fat stores.  The vagina will swell and lubricate in response to sexual arousal, stretch and grow in response to pregnancy and childbirth, and repair itself in response to birth or other trauma.

Vaginal secretions tend to be acidic and act as a cleaning solution to keep the balance of normal, healthy bacteria in check.  When presented with excess or foreign bacteria, the vagina will change secretions to clean itself out.  If bacteria grow out of balance, the result is a vaginal infection.  Yeast is a common fungal infection that grows well in the vagina because it is a closed, warm, dark, moist space.  

During and after menopause, the skin of the vagina and vulva becomes thinner.  Many women use a vaginal lubricant or even an estrogen replacement cream to counteract the dryness.  

Sexually speaking, the vagina continues to be responsive throughout a woman's life. There may be changes, but studies suggest that sexually active postmenopausal women have better vaginal circulation and muscle tone then those who do not engage in sexual intercourse.   

Resources:  
Archives of Gynecology and Obstetrics http://www.springerlink.com/content/p554l7wl44154217/

Family Doctor.org http://familydoctor.org/online/famdocen/home/women/reproductive/menopause/125.html