No doubt his power was what primarily attracted Lewinsky to him.  And she was 22 at the time, not 19. But whatdoes age have to do with it once one is a functioning adult and makes their own decisions? Granted, most people don't have the capacity to make good judgements until age 24-25, but that doesn't stop us from sending people off to war at age 18'does it? A person is considered legally an adult at age 18 in most states and is 100 % responsible for their own behavior.  They can sign contracts, get married, be charged tried and convicted of a crime. In fact, it not unusual for children far younger than the chronological and mental age of 18 to be incarcerated for life and/or executed.  
I'll bet all of that concern you have goes out the window when you read about some young person who commits a crime, right?  Then it's lock 'em up and throw away the key.