I finally stopped chewing my nails around age 25, when my now spouse nagged me to the point that I'd take my hand down from my mouth even when he wasn't there to see it. He also took over keeping them trimmed, so there weren't any rough edges to start chewing on.

I'm all for you quitting smoking too. I have never smoked, but as a child of a heavy smoker, I do tend to blame my tendancy to get upper resp. infections on all the second hand smoke that was in my house growing up. As a non-smoker, I don't beleive I give anyone dirty looks for smoking either though, but it may well depend on my state of mind and ability to get away from the smoke.