FORMULA AND VALIDATION RULE

create a Formula to calculate Age from Date of Birth

( TODAY() - DOB__C ) / 365.2425

create a validation rule Date of Birth Not equal to today

DOB__C = TODAY()

create a validation rule that Next visit (date type) should be greater than the created date +30 days after

Next_Visit__c  < (DATEVALUE( CreatedDate)  + 30)