Hello There are many causes of irregular periods. Stress, anxiety, weight loss or weight gain, emotional trauma is one reason. Hormonal imbalance due to thyroid disorders, adrenal disorders etc, or polycystic ovarian disease both are leading causes of irregular cycles. Again, some local pathology in the uterus or ovaries might cause these problems At the extremes of reproductive life, both age groups might naturally face irregular cycles. You should consult your gynecologist for a detailed evaluation and all investigations to pinpoint the cause of irregular periods. Take care