Is it possible for a 30 year old to have dimentia?

Unlikely: Anything that is conceivable is possible. A 30 year old person with Down syndrome could have dementia. A 30 year-old with genetic predisposition to Huntingtons disease could get dementia. In a normal regular person, it would so vanishingly rare as to be non-existent. Wish you good health! - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.