Someone really did not explain this well to you. The narrowing you are referring to is not between discs. It is where a disc is between two vertibrae. L5 stands for your fifth lumbar vertibra. S1 stands for Sacral one. The term L5,S1 refers to the disc between the two. What is probably happening is your disc is narrowing. When we are young, discs have more moisture. As we age, they dry out, so the tend to narrow. So you will almost certainly have additional narrowing of the space because you will get older and the disc will dry out. This may or may not be the cause of your pain. I am also curious who gave you the Xrays and where your pain is. I hope this is helpful. I am sorry to hear you are having problems. Especially someone so young. Good luck!
Based on what you have told me I would expect the best thing you could do right now is to keep yourself in good physical condition. Something like yoga or pilates would strenghthen your back muscles and abdominal muscles and improve flexability. This could be done very simply. A dvd or tape of something like pilates for dummys that could be done at home would be a good start. I am guessing that when you say "will something have to be done with it", you mean surgery. I think that is very unlikely in the near future. There is potential for problems many years down the road but if you keep yourself in good shape, you may be able to avoid it. I hope this comforts you a little. Backs can be scary and when you are told that something is wrong with yours, it is even more scary. Hang in there and keep us informed.
To the best of my knowledge, this is referred to as "stenosis". I have this too but it did not show up on regular x-ray (or so I was told), only on a MRI.
I am almost 42 and while it does cause me a great deal of pain at times, my doctor said no need for surgery.