Interesting questions, BDSM_Tourguide, but a couple of them should be no questions at all starting with:

At what age is a person sexually considered an adult?
A person may become sexually active, but that does not render anyone an adult. Being an adult means being able to assume legal responsabilities on behalf of oneself and most importantly on behalf of a dependant and that is more related to your second question

At what age is a person mentally considered an adult?
An this one is related closely to the personality of the people in question. Although immature and inexperienced, I considered myself mentally (although not legally) adult at 14 years old and I know people in their 20's and 30's who still have not reached that stage.

How do geographic, economic and societal stresses and influences affect your decision?
I have lived in 5 different countries and although societal expectations and stresses vary, I would have to say that adulthood can still be measured by the ability to legally become responsible for others. That may be easier in some societies than others but the concept holds true no matter where you are.

Should an age of consent apply to age of marital consent as well? If it should, how should the age of marital consent be affected?
Ideally it should apply but we are thinking here about an utopian world where sexually active people era actually educated and responsible.

If a worldwide age of consent were reached, how should it legally be enforced? Should there be exceptions in cases of parental approval? Should there be an exception if both parties are under the legal age of consent?
What a laugh. The USA cannot even enforce the legal drinking age, the Soviet Union could never enforce state-approved atheism and you are asking if we could enforce a world-wide age of consent? As for the second part of your question, let me ask: how many people do you think would ask for parental approval and how many parents would give it?

Given technological advances and new science, are today's laws adequate to deal with underage sexuality? What could be done to further improve the laws governing age of consent?
The problem with technology and laws is that they are utterly useless in dealing with the real question, which is the mental developpment of children and their education in the areas of social, civic, sexual and legal responsablility. The answer would be to improve the laws dealing with education.

Finally, in cases of underage sex leading to pregnancy, what should be the determined outcome of the fetus? Should all fetuses carried by illegally impregnated mothers be immediately terminated? Should the underage mother's parents decide what is best for the fetus? Should the underage mother be given a say in the matter? Whose responsibility should it be to raise the baby, if the fetus is allowed to go to term?
Ah, me... the abortion debate. Should all fetuses carried by illegally impregnated mothers be immediately terminated? And this would be done as punishment to the "illegal mother"? We are not even considering the religious, moral and emotional implications.
Should the underage mother's parents decide what is best for the fetus? That is, after failing to properly educate their own offspring? And if they did are they willing to become the baby's legal parents?
Should the underage mother be given a say in the matter? Sure, if she (and the father) are willing to become responsible for a newborn baby.
Whose responsibility should it be to raise the baby, if the fetus is allowed to go to term? Well here go again to the original quandary. Does consent imply responsability?

Consent to become sexually active can not be regulated, legislated or enforced. Legal ages for drinking, driving, voting, etc, are all subjective, arbitrary and ultimately unjust. They are however, the only things we have to avoid social chaos. There will always be people who are mature beyond their years and there will always be inmature morons who should be kept off the streets for their own protection and everyone else's. The real answer is education and parents giving their children a good example in responsability.

Now, want to see a crazy question? Should there be a legal age of consent to practice bdsm in a non-sexual way? How old do people have to be to legally say "please slap me, I think I will enjoy it"