I have heard differing opinions on this as well. I think the majority of people who sell pilot pitches and specs have experience. It makes sense if you think about it...having more knowledge and experience would likely result in a better pilot, and perhaps a more commercial idea. More experience also results in more connections and friends around town who want to work with you, which likely improves your chances of selling something.
Still, it's not impossible to sell an idea as a newbie. I know a young writing team, Hyatt & Umansky, that sold a drama pilot to NBC last year. Sure, networks and studios want to be in business with people who have heat around town, people their competitors are talking about...but each year, networks buy ideas from both established writers and newbies. They can't all be $500,000 ideas. Buyers also look for a variety of points of view, so they might take a chance on someone coming from features, comic books, theatre, stand-up/sketch comedy, etc. (Another friend of mine sold a pilot after his feature script landed on the Black List.) Non-TV-buzz also worked for Lena Dunham, who sold upcoming HBO series Girls after gaining heat from writing, directing and starring in features Tiny Furniture and Creative Nonfiction. Like many new TV writers, Lena was paired up with seasoned producers: Judd Apatow and Jenni Konner. Finding an established producer to help you develop and pitch your idea is definitely a way to help your chances.
Also remember that cable networks launching new scripted departments might be casting a wider net, and might be open to ideas (and writers) that other networks aren't. Matthew Weiner wrote Mad Men as a spec, and HBO and Showtime passed...but a few years later, it was a perfect fit for AMC. Weiner did have experience, yes...but I think many cable networks would be willing to take a chance on a new writer if the idea was really attractive to them.
All you need is a pitch meeting, or to get your spec pilot in the hands of the right people. Generally for either of those things you need a manager or agent, so you might want to start with that.
From my research, it seems that most shows on the air were created by people with at least staff writer experience... but remember that a bunch of ideas are purchased and then a bunch of pilots are shot before things get to the air. Also remember that spec pilots are increasingly desirable as writing samples to get you an agent or get you staffed on a show. At a beginning point in your career, I would think less about season 4 of your spec pilot series and more about how you can make the pilot script the best writing sample possible.
Also, I know that some new writers think, "I don't want to be staffed on someone else's show...I only want to work on my own ideas." This attitude isn't going to get you anywhere. If your ultimate goal is to be a TV writer, being staffed would teach you a world of things about writing - and yes, probably make the process of pitching your own pilot ideas easier.