Diagnosed infertility is not commonalthough on the increase (perhaps due to greater awareness), Sub fertility is far more common. It is possible to not actually be diagnosed as infertile yet still struggle to conceive. Sometimes this is referred to as unexplained infertility.
From the time you get your period (menarce) until the time you cease (menopause). So long as there are no other presenting problems one could argue she remains fertile all this time. However, fertility declines with age mostly because the quality of the egg (and to some extent the sperm) declines with age.
Women have what is called a fertile window, which is a few days in the middle of the month when she produces an egg from a follicle in her ovary. Often vagina secretions change and become like egg whites. If you have a 28-day cycle then you ovulate around day 14 (but this varies), Emma states that increasing sex from day 8 is advisable.