Will smoking marijuana lightly during teens cause developmental damage to the brain?

It is impossible to say with any degree of certainty whether a given individual will or will not suffer from any consequences of any amount of marijuana consumption, so we'll have to use the scientific data which can only say something about group-level effects.

It is quite well known that cannabis impairs cognition in the short-term*, and that the effects may last for at least a while of abstinence in heavy users. What you're asking about is whether there are any lasting and irreversible effects of cannabis on cognition, and this remains a debated and unresolved question in the scientific literature on harmful effects of cannabis. For instance, a recent review find inconsistent evidence for long-term working memory deficits in current heavy users and limited evidence for recovery after abstinence. In the same study, similar results are reported for other areas of cognition.

When it comes to IQ, there are some long-term studies that assess whether there are lasting long-term effects of cannabis on IQ. I believe the first such study was the one published by Meier and colleagues in 2012, in which they reported that among heavy users who had begun using cannabis before the age of 18, there was a lasting decline of IQ from childhood to mid-life with about 8 points, that persisted even after cessation of cannabis use. A similar study was published in 2016 by Mokrysz and colleagues, finding no decline of IQ between ages 8 to 15 in adolescents and pre-teens who had used cannabis during those years. Both studies may be subject to methodological criticism, and to my knowledge, there are no other studies with similar design.

Another issue is whether cannabis impacts brain development and/or causes morphological brain changes. The literature on this topic is also on the whole inconclusive, as shown in a recent review by Kroon and colleagues. However, it should be noted that there is actually some evidence ("limited evidence") for reduction in the volume of prefrontal cortex and hippocampus, which is consistent with animal studies in which especially a reduction of hippocampus volume can be induced under experimental conditions.

In short, the scientific literature in this area is on the whole inconclusive, and while it cannot be ruled out that there might be irreversible negative effects of cannabis on cognition, especially in those who start to use cannabis early and develop a heavy use over long periods of time, there are no hard evidence to support this either.

*. Though there is also some reports indicating positive effects of cannabis on divergent thinking (i.e. "out of the box"-thinking), which might be considered a positive effect on cognition.