Re: L5-S1 Disc almost completely gone...
Welcome to the board. You certainly are young to have this sort of back issue. Do you have immediate family with bad backs?
Degenerative disc disease is really a misnomer as it makes it sound like something that just continues to get worse and worse. As I'm sure you've discovered, there is tons of information about DDD on the internet...some of it better than others. Keep that in mind as you are reading.
Even though it sounds like a disease for "old" people, keep in mind that our spines are better suited for critters that walk on four legs. Our spines begin the aging process in our twenties, although for most of us, it is nothing noticeable.
As pooby suggested, the condition usually takes care of itself, with time...but it is painful in the process. Sometimes surgery is necessary if a spinal nerve becomes compressed in between the bones.
If you choose to get another opinion, particularly if your current doctor recommends surgery, you might want to look for a pediatric orthopedic surgeon. Your bones are still growing, so it might be important to have a pediatric surgeon who is used to working with this issue.