Can a 18 year old get vaginal cancer?

Very, very rarely: Vaginal cancer is rare, and usually found in older women (at least over the age of 30!) or uncommonly in very young girls (under the age of 8). Young adult women should think about preventing cervical cancer by getting vaccinated for hpv, and see a healthcare provider if you are have concerns about pain, discharge, lumps or bumps, or other changes in your vagina.

Vaginal cancer: It is very unlikely for an 18 year old to get cervical or vaginal cancer. Even the precancerous lesions are very unlikely unless you have some sort of immunosuppression (hiv, transplant ...).