Menarche is the time you menstruate for the first time.
This may happen when you are 9 years old, but also when you are 16. The mean age is somewhere between 12,5 and 13 years.
If you never had a period by the age of 16 you should visit a doctor. You may be just a little in your physical development, but you have to make sure nothing is wrong.
The minimal weight to start menstruating is around 46 kilo (101 pound). If a girl is much lighter periods may fail to come. Periods can also stop once they started but the girl looses too much weight.
At first menstruation may be irregular. It can take one or two years before the cycle becomes regular. Sometimes girls are anxious when they skip one or more periods. But at the start this is common and nothing to worry about (unless you had unsafe sex and may be pregnant).
If you have never had a period you should still use condoms when you have sex. Not only to be protected against STD’s, but also against unwanted pregnancy. Before you have your first period an egg cell may be released, leading to fertilization when you have intercourse.