Risk assessment is an interesting issue (I am still trying to dig around for those files...sorry its taking so long).
1. I think the desire and/or drive to mitigate risks comes from knowledge of those risks and perceived implications. I view the situation (in the context of the above post) as a modified prisoner's dilemma. A scuba instructor has an incentive (or probability) to defect (teach poorly, slack on standards, etc) unless there is a compelling reason not to. This compelling reason is formed by repeated interactions and the perceived importance of those interactions. An instructor is taught that failure to apply proper standards and the inability to stay up to date with current regulations is not only grounds for dismissal from the agency but legal action. This is especially important if instruction is their primary source of income. An 18 y/o just out of hs can understand this risk, instructor programs drill this into your head. Even when you go to the IE there is a huge orientation/ending dedicated to this very topic. Scuba instructors also have an incentive to teach well so they can continue to get business. By this logic (although a slow process), instructors that teach poorly are weeded out and left without students. This is checked by their ability to produce references.
2. The knowledge of risks is important and I don't think an 18 y/o is uniquely disadvantaged. I know plenty of old timers (25+) that cave dive without the training, have their computers beeping like mad after the dive, diving alone, etc. Their response when confronted was, 'never was a problem before,' or 'i'm gonna die sometime.' Many of the youngsters I encounter were raised on strict standards and rules nonexistent in the earlier years of diving. I don't want to be misunderstood here as I know that earlier dive training was intense and really just 1 cert, there wasn't 'another cert'...if my point needs clarification, let me know. Moreover, I don't think that the risk associated with diving is hard to understand...18 y/o are not necessarily babies.
3. The response/actual risk management I do not perceive a problem if the diver is experienced. Taking your time with the certifications, gaining experience in different types of diving, and dm'ing for awhile is definately a good idea before becoming an instructor. If the 18 y/o did all this, they should be well prepared for emergencies should they occur. Given, they won't be prepared for every emergency until it actually happens, but that is not unique to an 18 y/o. An interesting question to ask the instructor is how much time did they spend dm'ing/in training.
4. An instructor consistently participating in continuing education courses is an instructor that is excercising adequate risk managment. Many young instructors take in a lot of information and take advantage of their youth...plenty of older instructors may feel as though they don't have time or choose not to pursue further education.
As far as teaching is concerned:
1. The ability to teach is not unique to any age. I think the instructor's background/resume is important. For instance, if they were president of the debate team in hs, a coach, a writer, etc. their articulation and explanation could probably be better than a full time scuba instructor.
2. Instructor theory and knowledge at good IDC's really does drill in how to adequately perform skills and convey important information. Its a good idea to know where your instructor was certified.
3. I don't, as of yet, see any information that suggests inferior brain development results in an inability to teach better than a 30 y/o. My health policy prof is brilliant but he can't teach to save someones life (no pun intended), yet the 18 y/o sitting in front of the class brilliantly conveys some of the esoteric info to the class. A lot of our schooling (atleast mine) was very intense insofar as presentations and discussions are concerned, articulation is a skill required at many institutions (including hs).
4. I appreciate Nomad's response; he is very right to suggest that it depends on the individual at hand. If they can teach, they can teach. By can, I mean ability. I have seen or read nothing to suggest that 18 y/o's can't teach. Whether or not the instruction is good is dependent on many factors (listed above and in previous posts) and checked by 1. the dive center, 2. the organization, 3. references, 4. standards and procedures, 5. continuing education, etc.