It doesn't hurt to try. The salesman that my aunt bought her car from was 26 I believe and had a college degree in marketing and loved selling cars, so that's what he decided to do. I'm sure there's plenty of people that hate it, too.
At the dealership I worked at I had to read some agreement before I could work there, and it talked about the salary of a salesman. I don't remember exactly what it was, but it seemed pretty decent and of course that was not counting commissions. I guess it all depends on the dealership, though.
Typically the salesman got 1 day off during the week. They usually came in at sometime from 9:00-10:00 and were still there when I left (5:00), I assume they stayed until it closed.
The salesman we bought our cars from is now one of the sales managers there. He was successful and worked his way on up the ladder to the manager position.
I do think that knowing the product gives you a huge advantage. Knowing what you're talking about make it seems like you care about serving your customers and care about what you do for a living. I also think this will make whatever test/training you have to go through much easier, as you already know most of the info.
Like I already said, I don't think it would hurt to try if you are interested. Hell, you could be the next Buickman