Sacral dimple/fistula just appeared at 19 years old, what to do?

Pilonidal sinus: At 19 years of age this is likely a pilonidal sinus from a hair follicle affected by pubertal hormones, follicle becomes distended with keratin,a folliculitis develops, the infected follicle extends and ruptures into the subcutaneous tissue, forming a pilonidal abscess. This results in a sinus tract that leads deep into the skin. See his doctor for treatment.