Usually, it's about forming relationships with potential customers. It's then about maintaining those relationships in order to make sure they are staying satisfied with that they've bought or will buy.
They aim to sell the products the company they're working for offers.
They answer to customers' requests and look for new potential clients.
Usually, since it's easier to form relationships face-to-face, they're most of the time on the road. This being said, most salespeople also have a desk of their own somewhere.
Simply look for companies that are looking to hire salespeople. Though, to get the job, you'll have to make give a certain attention to how you're dressed, how you talk and how you walk.
It really depends on their sector of activities, but usually the ceiling is pretty high since it often pays on a commission model.
Knowledge of human psychology can be a big plus, but this knowledge can also be obtained simply by working in sales in itself or in customer service.
Some salespeople sell by using techniques that can sometimes be seen as un-ethical, most of the time when these techniques imply psychological tricks. That's the main reason why the job of salesperson is often seen by outsiders as a job that thrives from human naiveness.
Slim to none. Since a good seller relies on his interpersonal skills, robots simply cannot replace them, at least for now...
A salesperson does not really have any fixed retiring age : He can go on and do his job for as long as he wishes.
In conclusion, salesperson is a job that is often misunderstood, but in itself is a job that aims at satisfying the needs and wishes of people, which is in itself a case that's defendable.