[*]At what age is a person sexually considered an adult?

See next question, but a sexually active person should (as per RFC standard, aka MUST) be mentally active. (at least conscious of the implications). Moreover, it should be matured enough, from a physical point of view. This puts the lower limit to 16, at least.
[*]At what age is a person mentally considered an adult?

How does the mental of a person relate to his/her age ?
What about trisomic and supergenius ?
What is an adult ? If the answer is 'a person responsible and accountable for his/her acts',
[*]How do geographic, economic and societal stresses and influences affect your decision?

Ok. I just stick to my local law:
[list=A][*]responsible from 13[*]sexuality from 16 only with less than 18 old (more than 18 old could get prosecuted on younger or younger's parent/tutor requests)[*]sexuality from 18 only with more than 18.[*]vote from 18.[/list=A]
I have a few other motivations, but they are not age-related.
[*]Should an age of consent apply to age of marital conset as well? If it should, how should the age of marital consent be affected?

marital consent at 18. Exceptionnal wedding from 16 with less than 18 old only with both side parents/tutors approval, but they could just wait 2 years as well.
[*]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?

See previous answer. Exception ONLY if both are under 18, (and still past 16).
[*]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?

What does technology changes in physical evolution ?
Which law are you speaking of ?
Your question seems unfounded, unless you provide more material.
[*]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?

The mother-to-be decision should be final, once provided rational and unbiased information.
The mother's parent should not have any influence on the decision, and protection should be provided for that purpose.
Medical matter might enforces fact only to protect the mother-to-be, in case of real and excessive danger.
The mother-to-be should weight at least three possibilities:
[list=1][*]abortion[*]allowing the fetus to go to term and take care of the baby herself.[*]allowing the fetus to go to term and give the baby for adoption (anonymously, no choice of family, no official-link between the mother and the baby. At best, she could leave a letter for the baby to be given/opened only upon request of the baby when at least 18).[/list=1]
The father-to-be could at best support the mother-to-be (if he is known... there is still the bad case of rape...)