Hi. I'm a 19 year old girl. My upper incisors are about to fall at my age. Its not even decayed. What are the possible causes for this?

Periodontal Disease. You may be loosing the bone support of your teeth under the gums. You may have short roots and active gum disease. Its possible to loose supporting bone without loss of gum height around a tooth. See a dentist for an some xrays and a diagnosis. Periodontal disease is often not associated with decay.
Gum problems. It sounds like the bony support for the teeth is compromised. Usually this is because of really bad home care and/or physical trauma.