"However, some names are much more specific: “menstruation” is from Old English “mondablot” or “month blood;” in Latin, “menses” means “month;” ... (2); and the term “period” dates back to 1822 meaning “an interval of time”
A History of Menstruation: terms, customs/beliefs, and fashion!
"Menstrual - pertaining to menses," also (in astronomy) "monthly," from Old French menstruel, from Latin menstrualis "monthly," especially "of or having monthly courses," from menstruus "of a month, every month, monthly, pertaining to a month," from mensis "month" (see moon (n.))."
Online Etymology Dictionary
So Menstruation/Menses (essentially "monthlies") predates "periods" so you could argue that "periods" is the term come lately.
To actually answer the OP's question I found this useful, I've found the same information on plenty of other websites too:
How will I know when my periods are soon going to start? from NHS Choices
Signs that your period is on its way are if you’ve grown underarm and pubic hair. Typically, you’ll start your periods about two years after your breasts start growing and about a year after getting a white, vaginal discharge. The average girl will get her first period around 12 years old, but it varies from person to person.