Yes, anyone can have heart valve disease. It is unlikely though. Usually if a 12 year old has an abnormal heart valve, then it is secondary to a congenital heart defect (something they were born with.) Infection of the heart valve (endocarditis) can occur in any age group and lead to an abnormality in the function of the valve involved. Rheumatic heart disease is also possible, but not common in the developed world since the regular use of antibiotics. Good luck.