I just quit on Friday.
I'm almost 41 and I've been taking good care of myself except for smoking. I read this thread the past few weeks and thought about it seriously and realize that smoking does hinder my gains, it hinders my appetite, and isn't good for my already bad allergies. So I decided to just give it up.
Funny thing. My Father also quit smoking when he was 40 and never picked up a cig again.