Let's argue the positive aspects of keeping the smoking age at 18:
One less law for officers to enforce.
More freedom to do with your body as you wish.
Tobacco companies make more money.
More jobs are created treating people with lung cancer.
More people are removed from the workforce leaving openings for people.
People die earlier and thus don't collect social security in old age.
People can have fond memories of deceased relatives instead of the sometimes negative experiences in the present if a person lives.

What did I miss?