I got matured at my 18. It depends upon the body nature. It doesn't mean 15 years is really too old to get your period. Just relax and continue enjoying your life. You have a long way to go.. You would not wanna start thinking too much about these things now. The white discharge may also be another reason that your period is gonna happen soon..
Just echoing that, active females, women who have low percentage of body fat and also genetics may mean there is nothing wrong with a later onset of period. My best friend actually got hers at 16 (and boy was she sweating waiting on it) but she has never had anything wrong with her reproductive system and actually has 2 beautiful munchkins of her own now.
Now because it is kind of late you may want to get your dr's attention. Quite probable you're fine, but if you are from a family with 10-13 period onsets, have normal body fat range, and aren't extremely active then your dr should be aware of it just in case.
We all develop differently and at different times. Some girls who are very active in sports, especially girls who run cross country, tend to start their first period later than girls who aren't as active. It has something to do with amount of body fat they have. If this is you then this may explain why you don't have your period yet. Either way, it is a good idea to make an appointment with your doctor to find out what is going on with you.