thank u for ur enquery ur son have a lump under chin that is fixed to the underlying structure at the age of 18 yrs most common cause at this age group of a submandibular swelling is oral infection or dental caries...but in that case the swelling would be a bit painful and not fixed to the underlying structure so probability of it is less again this lump may be due to oral malignancy especially d/t tongue cancer..it ll be painless and hard and fixed to underlying structure...which is the case with ur son ...but tongue ca is rare in this age group or may be nasopharyngeal cancer....if it is the case then the disease has already progressed a lot...need early medical attention better to have a FNAC from that mass...if diagnosis is done then its ok if not done then excision biopsy should be done...that ll definitely make the diagnosis consult a surgeon immediately and manage accordingly need not to panic thank u