I grew up around a lot of people who were regular smokers from an early age, say 15. (and by regular I mean "can't sleep without pot"), and their cognitive abilities definitely suffered from smoking, and they freely admit it, and I think it's very obvious if you're around people who smoke a lot.

You simply can't concentrate when you're high the same way you can when you don't smoke. Reading is more difficult, exercise is more difficult, that's just the literal effects of weed.

If you're high all the time you're not going to perform the way you can when you're sober, and this is just my anecdotal observation, with people who started smoking young it becomes a sort of personality trait. You just become less sharp, less active, and when there's habit less interested in doing things not-high. Studies aside, that just can't be helpful.