So they have all gone through college and gotten some experience in their particular field. I don't know of any age requirement, but the experience that is needed would probably mean that the astronauts are at least 25, more likely over 30. I asked your question about what an astronaut wears to my friend, Ron. The EVA suits are the same with an added cooling garment that goes on over the long johns. It is like another pair of long johns with small plastic tubing woven all through it.
Chilled water is circulated through it to get rid of body heat and solar heat.
It depends on how well the spaceship is designed to handle the heat and light. Well, there isn't any pressure, because space is nearly a vacuum. That is one of the reasons that astronauts wear spacesuits. In order to breathe on the shuttle, they take along a supply of oxygen. There are also filters that take carbon dioxide out of the air.