When will my daughter's first period start? She is 15 years old, and all of her friends already have started.

I didn't get mine 'till I was about 15 and a half...and was delighted not to have it 'till then! A lot of girls get theirs very early nowadays and it's not necessarily better. I've heard that this can have something to do with diet. My mother was about 17 when she got hers.
Being from Europe, I am surprised how American girls can't wait to grow up. I was a very slim girl but healthy and got mine when I reached a certain weight. Is your daughter slim?
Why worry unless she is 18 or older? She is normal.

According to the American College of Gynecologists and Obstetricians (ACOG), "many girls have their first period around age 11 or 12 years, but periods can begin as early as 8 years or as late as age 16 years."

Another gauge: the American Academy of Pediatrics says, "Most girls get their periods 2 to 2½ years after their breasts start to grow (between 10–16 years of age)."

Of course, a quick call to your daughter's doctor could also ease your concern, but from what you describe, her age is within the range of what would be considered normal for first periods.