Well a number of things could be the reason to an irregular period. Your age and how long youv'e had your period are a one, it takes time for it to become regular. There are other reasons like hormonal imbalance/ diet/ medication
If you don't have any other symptoms then an irregular period can be a pretty normal thing. I had a pretty irregular period most of my life until I got on birth control and now I can predict the exact day that it will start every time!