They generally start their adult molt around 6 – 9 months of age. It is possible she is a bit older than you think – if she came from a pet shop it is easy to lose track of a bird’s age. If the weather has turned cold and you have started to run the heat and keep the house warmer, this can trigger a molt. Do you have a night light on for her at night? She might be having night frights, which some cockatiels have – for some reason they will thrash around during the night. Is she on a nutritionally balanced diet, or just seeds? If her nutritional needs are not met, this can cause feather issues. As long as she does not have any bald spots, then she is losing feathers naturally for some reason. If her diet it balanced, then try leaving a nightlight on for her at night. If this does not work, but otherwise she is eating well and seems healthy, then it is probably just her time to molt.