I started messing with guitars when I was 14. I got my first guitar on my 15th birthday which was also the first time I ever placed in a race (3rd in 800m 2:24 ugh!). I went on to become a very good runner. Unfortunately the guitar did not go so well when I was a kid. Having a good teacher is essential. My teacher then was a great player but not a great teacher. I could not take lessons because the music store closed at 5 and track practice ended at about the same time.
Fast forward many years, I started playing again 2 years ago at 40, taking lessons weekly from a very good teacher. I practice about an hour a day and I am still making great progress all the time.
I thought I could play some songs a year ago, but I have found that the better I get, the less I think I am actually "playing" songs since I keep gaining an understanding of how poor I sound compared to what a really good player sounds like. Similar to running, it seems the more I improve, the farther away from truly great I feel.
One piece of advice I would give is to play slowly and cleanly, always trying to perfect your tone. The more you practice, the faster you will be able to play but you don't want to be fast and sloppy.
Here is a great free lesson from Steve Vai, that covers very well the progression from the very beginning (deciding to play guitar) to the quite advanced (developng your own style). Gives you a good outline on how to progress from A to Z.
http://www.youtube.com/watch?v=j7pJ6jeXTT4
My teacher uses pretty much the same philosophy so I think this is a good overview for anyone interested in learning to play.