Possibility of stomach cancer at 18 years age? No family history of cancer, no alcohol, no tobacoo.

Very unlikely. If you have some symptoms, problems or dietary changes- talk to your family doc or see an internist. They can help you determine the problem. Cancer in an 18 yo is so unlikely without risk factors, it's as close to zero as it can get. Risks include diet, nationality, certain prior illnesses. Discuss all this w pcp.