Validation Rule for minimum age at least 18 and maximum age should not be more than 100 years in Salesforce
OR(TODAY() < ADDMONTHS( Age__c ,216), TODAY() > ADDMONTHS( Age__c ,1200))
Note: Where Age__c is Date type field.
Why we use 216 & 1200 : 216/12 = 18 And 1200/12 = 100