look at the impact on brain development. what do you make of that age of 21 being the cut off? >> during development, lots of things happen to the brain. it's unfortunate that we don't have a definitive time at which we know the brain is finished developing and it's clear you can do whatever you'd like. while the brain is developing the frontal cortex is the last to come online. and to be fully developed. the most important for things like regulating impulsivity, we already know that lots of emerging adults have difficulties making good decisions or not being impulsive. if you add to that different types of drugs or alcohol, it's going to be more difficult to behave appropriately. and what we have seen is some longer term impact in terms of brain structure and function in people who start smoking early versus those who started smoking later. we like to say it's worth the wait. just give the brain time to develop before you get into this.