I know it's taken me a while to respond but it's because I love this post, these are very good points that provide a good deal of fuel for thought, I would like to say that while I don't necessarily agree with certain things you've said, I think this is something that people really need to put a good deal of consideration into as a whole;

is it inherently damaging if it does not have a significantly negative, life impacting effect on the individual?

While objectively speaking, I would answer your question "is poor short term memory really damage" with the follwing: If the poor short term memory is a result of interference through substance, where otherwise it would not be poor, then objectively speaking, it is 'damaged' or at least, negatively impacted

in the context you've laid out, one could argue it's not necessarily damaging to the individual as a whole, and this is one of the broader context things we really need to consider and be careful of when using words like 'damaging',

as for 'arbitrarily assuming' that altering cognitive function would impede maturation,

I'd suggest I don't arbitrarily assume that simply altering cognitive function in and of it self is what could impede maturation, rather, that the effects of chronic exposure to a mind altering substance that significantly impacts the way you think and feel can interfere with and impede the process of maturity as it's, likely possible that it will have a large effect on how you process life experiences, on how you learn to cope with, deal with, apply yourself and live your life in the context of sobriety; I would suggest furthermore that this is not necessarily going to be a negative impact, but that in the case of cannabis and the way it effects the mind (cannabinoids) it will more than likely offset the process through an almost delaying effect if consumed too regularly and in large enough amounts.

Bare in mind, this does not necessarily mean it will be the case 100% of the time, but it's something we should take into consideration and look further into if we wish to understand the potential negatives from a scope beyond simply 'is it damaging', much like the above. Positive and negative effects, what are those, can we possibly find patterns that lead to significantly negative effects and, if so, how can we use that information to shape the way we approach cannabis as a society through adequate information, not misinformation