I was hired by a private service at 19 and a freshmen in college, so I only work during breaks/vacations. Something that helped me was being a volly for 6 years, yes, I started when I was 13 on the FD. This experience and driving firetrucks for a year and a half prior to hiring helped since most ambulance companies require people to be 21 to drive ambulances in the area. That is somewhere that I ran into trouble but because of experience and driving record Im now one of the preferred drivers on long transports. I was aided because the Chief of my FD knew the Manager of the ambulance service. So knowing people helps, because then you get to prove yourself on the job once you get it.
They would want to hire someone knew because it helps to have not just more seasoned people on departments, because when they go, who is left? Also, new EMT's generally know their stuff, key word generally, which helps to keep the protocols intact. As the saying goes, ALS without BLS is BS.
Good luck, Im sure that if you know your stuff you'll be fine and get hired.