Aside from the physical effects, quite possibly long-term but not conclusively proven to be the case (afaik), one reason to be extra-wary of heavy smoking < 25

Aside from the physical effects, quite possibly long-term but not conclusively proven to be the case (afaik), one reason to be extra-wary of heavy smoking < 25 is really all the usual stuff: lethargy, developing addiction/dependency, social and career stagnation. These can have a pretty negative effect at any age, but at a younger age the effects compound and end up much worse. The things you learn and experience in your twenties are often crucial preparation for how you'll doing in your thirties and beyond. If much of that time is spent being high and doing little, you'll pay the price later on. Basically, my experience (personal, and looking at friends and acquaintances) is that chronic, daily smoking often results in being 'stuck in place'. Getting stuck in the jobs, friendships, experiences, and relationships that dominate your early twenties is probably not a desirable outcome. On the other hand, a seventy-year-old picking up a weed habit probably won't suffer much from it. At any point in between I'd still argue it's better not to chronically dull your brain, but hey if you're happy do whatever you want! That said, obviously this doesn't apply to everyone. But I know a number of daily smokers who started early, and with few exceptions they are not very happy or successful thirty-somethings (by any metric they or I care about).