There haven't been any rigorous scientific developmental studies in humans, which is understandable given that pot is illegal. They have been able to show that heavy (many times a day) long term users are dumber, have worse memory, are less advanaced in their careers, and are more depressed than the general population, but there's always a chicken and egg issue there. Few happy productive people are going to find times to smoke many x per day, and few stupid people are happy and productive.
In rat studies, juvenile rats exposed to fairly substantial doses of the drug showed moderate, behaviorally significant deficits in short term memory, motor control, and I think sensory gating (i.e. properly filtering out obnoxious noise/visual stimuli). These effects were permanent, observed long after the drug had left their systems. Adult rats who were exposed in the same dosage regimen did not demonstrate these effects in the long term, although there was a similar response profile for a day or two after.
There is always the problem of relating animals to humans, as well as determining equivalent ages for rats and humans. I remember trying to work it out myself and thinking at the time that a 10 year old who smoked 4 or 5 joints a day for a few years would probably cause some permanent issues. A habitually using 16 year old? I don't know. If you're 16 and you're only smoking a couple of times a week the effects will be minimal at most, probably not noticable if you're already drinking/getting little sleep.
