My general practitioner is about 40 years old and i'm thinking of changing to a geriatrician since my age is 72. what is the professional organization for this as i'm searching for a board certified one in this field (online search goes nowhere)?

If possible you should be taken cared by a general internist or a geriatrician. In general, a 40-year-old family doctor has enough clinical experience (more than 10 years in practice if he or she is an US graduate) in general practice but not perfect for adult medicine. If you know the full name of the doctor then you can go to website American Board of Internal Medicine (ABIM) for more info.