Exactly that. It's hard to have any sort of control over a lifetime, but I went to university, have worked in software, and some of the friends I compare myself to did the same. I'm still "smart". But I feel like smoking as much as I did has accelerated my age-related cognitive decline.

I don't feel that I wasted the years, but I do think that the thousands of hours spent stoned were a complete waste and have decreased the potential quality of the latter half of my life.